fix#446
* Read signatures no longer include `opcode`
* stream::got_binary and stream::got_text inform the caller if
the current incoming message is binary or text.
Actions Required:
* Remove the `opcode` reference parameter from calls to synchronous
and asynchronous read functions, replace the logic with calls to
stream::got_binary and stream::got_text instead.
fix#446
* message_type is removed
Actions Required:
* Change call sites which use message_type with set_option
to call stream::binary or stream::text instead.
Vinnie Falco