GitHub commit count

Refactor current logic to use github api and auth token.

- Using authentication increases the rate limit from 60 to 5,000 requests an hour.
- auth_token needs to be created. See settings.local.yml.example. See https://help.github.com/articles/creating-an-access-token-for-command-line-use/ for creating auth token.


- https://www.pivotaltracker.com/story/show/78678240
- https://www.pivotaltracker.com/story/show/64891298
- https://www.pivotaltracker.com/story/show/80236842
it would be better to use `inject(0)` to explicitly define the initial value used for the sum