Added SkipTLS option to config.

Sometimes it is required to skip TLS as part of the XMPP protocol.
For instance, Google's GCM assumes TLS to be set up before the first message is sent. This can be achieved by setting up the tls connection manually and use xmpp.Config's Conn field. However, the xmpp package then fails with errors.New("xmpp: server doesn't support TLS").