Initial Win32 support.

This library now compiles on Windows but requires VS2015 for Windows.
VS2013 may work but this has not been tested yet.
Requires NOMINMAX to be defined to avoid clashes with the Windows.h
headers and the C++ stl and this libraries Date max functions.
Also known to work equivalently with recent g++ and mingw combinations
but -std=C++14 flag should be set.
C++14 restrictions may be relaxed later. Patches welcome!
g++ 5.1 was tested. It may work with other version but that hasn't been tried.
Mac OS and Linux systems are known to compile with -std=c++11
current_timezone and locate_zone will return iana names not windows
time zone names. This is expected and as designed.

tz.h

@@ -11,6 +11,30 @@
 // 1.  Reload database.
 // 4.  Is the utc to sys renaming complete?  Was it done correctly?
 
+/*
+The notion of "current timezone" is something the operating system is expected
+to "just know". How it knows this is system specific. It's often a value
+set by the user at OS intallation time and recorded by the OS somewhere.
+On Linux and Mac systems. the current timezone name is obtained by looking at
+the name or contents of a particular file on disk.
+On Windows the current timzeone name comes from the registry.
+But however the name is obtained there is no guarante
+that the "native" current timezone name obtained in this way
+will match any of the "Standard" names in this library's "database".
+On Linux, the names usually do seem to match so mapping functions to map from
+native to "Standard" are typically NOT required.
+On Windows, the names are never Standard mapping is always required.
+One should not equate the mapping process with Windows.
+Windows is just currently the only client of them.
+Technically any OS may required mapping.
+*/
+
+#ifdef _WIN32
+#ifndef TIMEZONE_MAPPING
+#define TIMEZONE_MAPPING 1
+#endif
+#endif
+
 #include "date.h"
 
 #include <algorithm>
@@ -462,12 +486,77 @@ operator>=(const std::chrono::time_point<std::chrono::system_clock, Duration>& x
     return !(x < y);
 }
 
+#if TIMEZONE_MAPPING
+
+// TODO! Ensure all these types aren't exposed.
+
+// The time zone mapping is modelled after this data file:
+// http://unicode.org/repos/cldr/trunk/common/supplemental/windowsZones.xml
+// and the field names match the element names from the mapZone element
+// of windowsZones.xml.
+// The website displays this file here:
+// http://www.unicode.org/cldr/charts/latest/supplemental/zone_tzid.html
+// The html view is sorted before being displayed but is otherwise the same
+// There is a mapping between the os centric view (in this case windows)
+// the html displays uses and the generic view the xml file.
+// That mapping is this:
+// display column "windows" -> xml field "other".
+// display column "region"  -> xml field "territory".
+// display column "tzid"    -> xml field "type".
+// This structure uses the generic terminology because it could be
+// used to to support other os/native name conversions, not just windows,
+// and using the same generic names helps retain the connection to the
+// origin of the data that we are using.
+struct timezone_mapping
+{
+    timezone_mapping(const char* other, const char* territory, const char* type)
+        : other(other), territory(territory), type(type)
+    {
+    }
+    timezone_mapping() = default;
+    std::string other;
+    std::string territory;
+    std::string type;
+};
+
+#if 0
+// This represents the type for the tzi field in the windows registry.
+// It's TBD if we need this yet.
+struct TZI
+{
+    TZI() = default;
+    int Bias;
+    int StandardBias;
+    int DaylightBias;
+    SYSTEMTIME StandardDate;
+    SYSTEMTIME DaylightDate;
+};
+#endif
+
+struct timezone_info
+{
+    timezone_info() = default;
+    std::string timezone_id;
+    std::string standard_name;
+#if 0 // TBD
+    std::string display_name;
+    TZI tzi;
+#endif
+};
+
+#endif
+
 struct TZ_DB
 {
     std::vector<Zone> zones;
     std::vector<Link> links;
     std::vector<Leap> leaps;
     std::vector<Rule> rules;
+#if TIMEZONE_MAPPING
+    // TODO! These need some protection.
+    std::vector<timezone_mapping> mappings;
+    std::vector<timezone_info> native_zones;
+#endif
     
     TZ_DB() = default;
     TZ_DB(TZ_DB&&) = default;
@@ -481,6 +570,11 @@ const TZ_DB& reload_tzdb();
 const TZ_DB& reload_tzdb(const std::string& new_install);
 
 const Zone* locate_zone(const std::string& tz_name);
+#ifdef TZ_TEST
+#ifdef _WIN32
+const Zone* locate_native_zone(const std::string& native_tz_name);
+#endif
+#endif
 const Zone* current_timezone();
 
 class utc_clock