New HTTP interfaces (API Change):

fix #123
fix #154
fix #265

This completely replaces the HTTP parser used to read and
parse incoming messages. The new version is faster than
the old one, and written in fewer lines. A summary of
changes:

* parse and async_parse renamed to read and async_read

* basic_parser is optimized for linear buffers,
  use flat_streambuf for the best performance with these
  functions:

  - http::read
  - http::read_some
  - http::async_read
  - http::async_read_some

* The overloads of read() and async_read() which take
  just a header have been removed, since they would
  throw away important parse metadata.

* The derived class callbacks for basic_parser have
  been streamlined. All strings passed to callbacks
  are presented in their entirety, instead of being
  provided in pieces.

These changes allow use-cases that were previously
difficult or impossible, such as:

- Efficient relaying

- Late body type commitment

- Expect: 100-continue handling

test/websocket/stream.cpp

@@ -174,7 +174,7 @@ public:
             for(n = 0; n < limit; ++n)
             {
                 // valid
-                http::request<http::empty_body> req;
+                http::request_header req;
                 req.method = "GET";
                 req.url = "/";
                 req.version = 11;