fix#80, #212, fix#303, fix#314, fix#317
websocket::stream now provides the following families of
functions for performing handshakes:
When operating in the server role:
* stream::accept
* stream::accept_ex
* stream::async_accept
* stream::async_accept_ex
When operating in the client role:
* stream::handshake
* stream::handshake_ex
* stream::async_handshake
* stream::async_handshake_ex
Member functions ending with "_ex" allow an additional
RequestDecorator parameter (for the accept family of
functions) or ResponseDecorator parameter (for the
handshake family of functions).
The decorator is called to optionally modify the contents
of the HTTP request or HTTP response object generated by
the implementation, before the message is sent. This
permits callers to set the User-Agent or Server fields,
add or modify HTTP fields related to subprotocols, or
perform any required transformation of the HTTP message
for application-specific needs.
The handshake() family of functions now have an additional
set of overloads accepting a parameter of type response_type&,
allowing the caller to receive the HTTP Response to the
Upgrade handshake. This permits inspection of the response
to handle things like subprotocols, authentication, or
other application-specific needs.
The new implementation does not require any state to be
stored in the stream object. Therefore, websocket::stream
objects are now smaller in size.
The overload of set_option for setting a decorator on the
stream is removed. The only way to set decorators now is
with a suitable overload of accept or handshake.
