Added ability to get tweets by list timeline

This commit introduces the ability to fetch tweets using the [Twitter list timeline API](https://dev.twitter.com/docs/api/1.1/get/lists/statuses).

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).