XMPP python tests

Added test case for XMPP that does the following:

1) Send a message
2) Receive that message

Uses the Datastore to save a XMPPMetadata entry that indicates whether or not we've (1) not saved anything yet, (2) sent the message, or (3) received the message. Fails on the SDK because the SDK doesn't send or receive XMPP messages, but passes on App Engine and AppScale.