Refactor HTTP identifier names (API Change):

fix #171

Several names and  HTTP identifiers are renamed to be
more consistent, self-explanatory, and concise:

* "Fields" is a collection of HTTP header fields (rfc7230 section 3.2)
* "Header" is the Start Line plus Fields. Another way to look at it is,
  the HTTP message minus the body.
* `basic_fields` replaces `basic_headers`
* `fields` replaces `headers`
* `Fields` replaces `Headers` in template parameter lists
* `header` replaces `message_headers`
* `header::fields` replaces `message_headers::fields`

The changes are cosmetic and do not affect run-time behavior.

test/http/streambuf_body.cpp

@@ -9,7 +9,7 @@
 #include <beast/http/streambuf_body.hpp>
 
 #include <beast/core/to_string.hpp>
-#include <beast/http/headers.hpp>
+#include <beast/http/fields.hpp>
 #include <beast/http/parser_v1.hpp>
 #include <beast/http/read.hpp>
 #include <beast/http/write.hpp>
@@ -34,7 +34,7 @@ public:
             "\r\n"
             "xyz";
         test::string_stream ss(ios_, s);
-        parser_v1<false, streambuf_body, headers> p;
+        parser_v1<false, streambuf_body, fields> p;
         streambuf sb;
         parse(ss, sb, p);
         BEAST_EXPECT(to_string(p.get().body.data()) == "xyz");