Simplify API

Drops the Connection newtype, and shows TMVar Connection instead.

Hides Connection from Network.Xmpp, as the vast majority of users will
not need to work with Connection directly. The related functions are
now instead available from Network.Xmpp.Basic.

Renames `simpleConnect' to `session', and makes it flexible in terms
of authentication and whether or not to use TLS.

Adds some minor documentation changes.

We will need to export some session related information (such as the
acquired resource, stream properties, etc.). We will also need to
expose any failures encountered, probably by making `session' an
ErrorT calculation.

Also removed the Errors module from the Cabal file.