Added test for leaderboard page

This addresses the issue #63. 

Please review and suggest changes. The test passes locally for me. 

Thanks.
A slight design tweak -- we shouldn't hardcode urls like this. In this case we want to ensure the test walks through a workflow of `user visits site` --> `logs in` --> `clicks leaderboard link in navbar` --> `verifies stuff on leaderboard page`.

My suggestion, add a method in `base.py` that returns a Leaderboard object