Use ElementTree instead of domish.

domish is not intended as a general XML library it is intended to generate
the specialized subset of XML used by XMPP.

This also refactors XML generation to operate on the eventDict instead of
just the failure which will later gives us the opportunity to propogate structed
data in the eventDict (such as request params and session data.)

We also add trial unit tests for the new XML generation.
Merge pull request #4 from dreid/etree-xml-generation

Use ElementTree instead of domish.