Proving once again that untested code is broken code, correctly set two
attributes during initialization of Invitation. Increase coverage by
checking every attribute.
With the Python 2.7 deadline fast approaching, modernize the codebase
making use of the modernize module to switch to using six, as well as
other upcoming features, such as absolute imports . Stop using 2to3
for Travis, yay!
Invitations contain information about the user they are for, and
which user invited them, so export that too.
During invesigating attmepting to test the new attributes of Invitation,
it was discovered that there are no code paths to fetch them, so add
AuthenticatedUser.get_invitations(). The test added can do double duty.
Steve Kowalik