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_crawl.cpp

@@ -40,9 +40,9 @@ int main(int, char const*[])
             req.method = "GET";
             req.url = "/";
             req.version = 11;
-            req.headers.insert("Host", host + std::string(":") +
+            req.fields.insert("Host", host + std::string(":") +
                 boost::lexical_cast<std::string>(ep.port()));
-            req.headers.insert("User-Agent", "beast/http");
+            req.fields.insert("User-Agent", "beast/http");
             prepare(req);
             write(sock, req);
             response<string_body> res;