updated readme; minor bugfixes

updated README.md
bugfix: username property for turn is not yet available in chrome M27; workaround is to use user@host in the turn uri until M28 is around.
bugfix: dtls constraint was not passed from adapter to session manager
bugfix: receiving an error response for a session-initate closes the peerconnection
bugfix: non-standard mapping of a=ssrc: lines is now living under its own namespace.