Expose disallowTLS option in client.

This is needed for some Jabber servers (OpenFire), where the connection will
hang when trying to establish a TLS handshake.
Added HUBOT_XMPP_DISALLOW_TLS option to README.
Merge pull request #54 from Nevon/disallow-tls

Expose disallowTLS option in client.

Fixes #34