Facebook auth

This implements Facebook OAuth flow as per https://developers.facebook.com/docs/chat/

Usage example:
```erlang
Session = exmpp_session:start_link({1, 0}),
JID = exmpp_jid:make("john.doe", "chat.facebook.com", random),
Token = "AAASOMETOKEN",
AppId = "1234567890",
exmpp_session:auth_info(Session, JID, Token, AppId),
Connect = exmpp_session:connect_TCP(Session, Domain, 5222, []),
exmpp_session:login(Session, "X-FACEBOOK-PLATFORM"),
```