Merge new changes to unordered & hash.

- Unordered tests can run lightweight test or Boost.Test (at least
   theoretically).
 - Workaround Open BSD's incorrect numeric_limits.
 - Move the hash extensions in their own file.
 - Various small improvements to the unordered docs.
 - Fix some unordered examples.

Merged revisions 43117-43837 via svnmerge from 
https://svn.boost.org/svn/boost/branches/unordered/trunk


[SVN r43838]

examples/hash_functions/fnv-1.hpp

@@ -7,7 +7,7 @@ namespace hash
     template <std::size_t FnvPrime, std::size_t OffsetBias>
     struct basic_fnv_1
     {
-        std::size_t operator()(std::string const& text)
+        std::size_t operator()(std::string const& text) const
         {
             std::size_t hash = OffsetBias;
             for(std::string::const_iterator it = text.begin(), end = text.end();
@@ -24,7 +24,7 @@ namespace hash
     template <std::size_t FnvPrime, std::size_t OffsetBias>
     struct basic_fnv_1a
     {
-        std::size_t operator()(std::string const& text)
+        std::size_t operator()(std::string const& text) const
         {
             std::size_t hash = OffsetBias;
             for(std::string::const_iterator it = text.begin(), end = text.end();