Added ability to get tweets by list timeline

[barnabywalters]

This commit introduces the ability to fetch tweets using the Twitter list timeline API.

The group_id argument to get_activities() can now be an arbitrary string, which is used as the slug for a list. If no user_id is given then the current API user screenname is used to search for lists. If a user_id is given, it is used as the owner_screen_name query param, allowing one API user to get results for lists created by other users.

Though barely any new functionality was added in this commit I attempted to add a new test which checks that the correct URL gets requested. Unfortunately I cannot get it to pass because 'https://api.twitter.com/1.1/account/verify_credentials.json' gets called first for some reason. I have not used mox for testing before and would appreciate any help getting this test to run.

(Background: my personal use case for this functionality is to make a list of everyone on twitter I follow who doesn’t have a microformats-enabled site and follow them all through one PuSH subscription rather than separate ones for each user).

[barnabywalters]

Thanks for the help in IRC, I split the test into explicit/implicit user_id tests, both of which now pass.