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/parse_error.ipp

@@ -50,7 +50,7 @@ public:
         case parse_error::invalid_chunk_size: return "invalid chunk size";
         case parse_error::invalid_ext_name: return "invalid ext name";
         case parse_error::invalid_ext_val: return "invalid ext val";
-        case parse_error::headers_too_big: return "headers size limit exceeded";
+        case parse_error::header_too_big: return "header size limit exceeded";
         case parse_error::body_too_big: return "body size limit exceeded";
         default:
         case parse_error::short_read: return "unexpected end of data";