Correctly pass timeout to Queue.get()

Else we may hang indefinitely :) Fixes #122
Allow changing sender of messages in test backend

Instead of completely hardcoding which jid messages are sent from, add a
sender attribute to the TestBackend class which may be changed to change
where messages are coming from.
Allow passing block to popMessage()
Merge pull request #123 from zoni/tests

Improve test framework. Nice simple one