[WIP] Merge all Pull Requests in a repo

Closes #75

I wanted to see if this was the right direction for what you had in mind. If so I can finish it up by writing tests.

Let me know!
Two issues here:

* In [the test failure](https://travis-ci.org/education/teachers_pet/jobs/28456393), it looks like the URL you provide here is just missing the query parameters.
* In order for the rest of the test to work, this API call will need to "return" the hashes for the pull requests that get stubbed below.  Take a look at `stub_get_json` and https://developer.github.com/v3/pulls/#list-pull-requests.