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.

include/beast/http/impl/basic_parser_v1.ipp

@@ -5,6 +5,16 @@
 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 //
 
+#ifndef BEAST_HTTP_IMPL_BASIC_PARSER_V1_IPP
+#define BEAST_HTTP_IMPL_BASIC_PARSER_V1_IPP
+
+#include <beast/http/detail/rfc7230.hpp>
+#include <beast/core/buffer_concepts.hpp>
+#include <boost/assert.hpp>
+
+namespace beast {
+namespace http {
+
 /* Based on src/http/ngx_http_parse.c from NGINX copyright Igor Sysoev
  *
  * Additional changes are licensed under the same terms as NGINX and
@@ -28,21 +38,10 @@
  * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
  * IN THE SOFTWARE.
  */
-/*
-   This code is a modified version of nodejs/http-parser, copyright above:
-    https://github.com/nodejs/http-parser
+/* This code is a modified version of nodejs/http-parser, copyright above:
+   https://github.com/nodejs/http-parser
 */
 
-#ifndef BEAST_HTTP_IMPL_BASIC_PARSER_V1_IPP
-#define BEAST_HTTP_IMPL_BASIC_PARSER_V1_IPP
-
-#include <beast/http/detail/rfc7230.hpp>
-#include <beast/core/buffer_concepts.hpp>
-#include <boost/assert.hpp>
-
-namespace beast {
-namespace http {
-
 template<bool isRequest, class Derived>
 basic_parser_v1<isRequest, Derived>::
 basic_parser_v1()