add STARTTLS support to TCP connections

- Add InsecureAllowUnencryptedAuth to options; go-xmpp will not leak
authentication in plaintext over TCP connections (breaks compatibility
with previous versions, slightly.)
fails.
- Add StartTLS to options; go-xmpp will use STARTTLS if the server
requires it or the user requests it.
- Add IsEncrypted method so sensitive clients can check if the
connection is TLS encrypted.