Add subprotocol box.
Review URL: https://codereview.appspot.com/7017043

git-svn-id: http://pywebsocket.googlecode.com/svn/trunk@719 4ff78f4a-9131-11de-b045-6380ec9940d4
Add extra_headers attribute to request object to allow users to append their
own headers to the WebSocket handshake.

BUG= http://code.google.com/p/pywebsocket/issues/detail?id=127
Review URL: https://codereview.appspot.com/7197056

git-svn-id: http://pywebsocket.googlecode.com/svn/trunk@720 4ff78f4a-9131-11de-b045-6380ec9940d4