Export various types

Types that are exported are:

Network.Xmpp:
* SaslFailure (..)
* StanzaHandler (..)

Network.Xmpp.Internal
* ConnectionState (..)
* Stanza (..)
* TlsBehaviour (..)

Network.Xmpp.IM
* InstantMessage (..)
* Subscription (..)

Fixes #10.
Fixes #12.
Fixes #16.