CYB-41 Mock Data is fetched and shown on the Leaderboard

1. Added AngularJS leaderboard controller and factory, as well as integrated @ericayin 's front-end leaderboard html, css, and javascript code.
2. Leaderboard Factory fetches data from a URL (dummy JSON) and displays on front-end
3. Also included some small style issues not covered in https://github.com/ets-berkeley-edu/datacultures/pull/3
4. HTTParty and Canvas_lti_controller changes are included to show how a Canvas API call can be made to return JSON
5. Also implemented the config/secrets.yml functionality of Rails 4.1: Tokens are now stored there

https://jira.ets.berkeley.edu/jira/browse/CYB-41
@NolanChan Can we consolidate the `add` and `subtract` functions a bit, to make it more DRY?