fix#404
Parsing and serialization interfaces have been fine tuned and unified.
For parsing these stream operations are defined:
* read
* read_header
* read_some
* async_read
* async_read_header
* async_read_some
For serialization these stream operations are defined:
* write
* write_header
* write_some
* async_write
* async_write_header
* async_write_some
serializer interface is changed to be buffer-only, no streams,
and placed in its own header file.
Operations on serializers are moved to free functions as part
of the HTTP write family of synchronous and asynchronous algorithms.
Vinnie Falco