Added param `include_missing` to ranked_in_list()

To determine whether or not to include members that are missing in the results.
Use case: If I have a list of friends, some of which have no score (aren't on the leaderboard) - I don't want to see them in the list of ranked users. Enter `include_missing`!
Adding `reverse` to options for ranked_in_list()
Actually going to use **options for passing `include_missing`
Merge remote-tracking branch 'fork/feature/ranked_in_list_include_missing'

Conflicts:
	leaderboard/__init__.py
	setup.py
Merge remote-tracking branch 'fork/feature/ranked_in_list_reverse'

Conflicts:
	leaderboard/__init__.py
	setup.py