add STARTTLS support to TCP connections
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.

pranavraja/go-xmpp,ordbogen/go-xmpp,nb-titan/go-xmpp,skiz/go-xmpp,titan-x/go-xmpp,eagafonov/go-xmpp,mattn/go-xmpp,dullgiulio/go-xmpp,Like-all/go-xmpp