[Reviewer: Mike] Check Sec-WebSocket-Protocol header and echo it back if valid (fixes #252)

Mike,

Please can you review my fix to issue #252, which covered WebSockets support being broken?  The fix was simply to check the Sec-WebSocket-Protocol header and echo it back if it contains "sip".  There are no UTs for WebSockets (although I've run the UTs to make sure I haven't accidentally broken anything else in the meantime).  I have tested live in Google Chrome and successfully registered with both sipML5 and JsSIP.

Matt