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
Core:
* Test buffer_cat iterator move members
HTTP:
* Fixed yield / resume in writer
* Fixed message serialization with chunked encoding
* Test yield / resume in writer
* Test all conditional branches during message serialization
* Test chunked encoding
* Increase coverage on parse_error
* Add parse_error::general
WebSocket:
* Add error::general
* Increase coverage in error