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.

examples/http_example.cpp

@@ -26,9 +26,9 @@ int main()
     req.method = "GET";
     req.url = "/";
     req.version = 11;
-    req.headers.replace("Host", host + ":" +
+    req.fields.replace("Host", host + ":" +
         boost::lexical_cast<std::string>(sock.remote_endpoint().port()));
-    req.headers.replace("User-Agent", "Beast");
+    req.fields.replace("User-Agent", "Beast");
     beast::http::prepare(req);
     beast::http::write(sock, req);