Listen for Github pull request events and build them

[ferrouswheel]

Currently we only support building and testing the master branch. We should:

• Isolate builds - don't reuse the same dir for cloning the repo, support different remotes, and branch specific builds.
• Listen for pull request events to trigger builds.
• Update pull request with success/failure of build whenever the pull request commits are updated.

Requires #27

[holic]

For adding commit statuses, Baleen will need its own GitHub application key that is (OAuth) authenticated with a GitHub account that has access to the repo in question. There's a special scope called repo:status specifically for status updates.

Since Baleen needs to pull down the whole repo, maybe there's a way to combine this access request with the full repo scope so that you don't have to add an SSH key for Baleen?

[ferrouswheel]

It would make sense to just use a github application key rather than also require ssh deploy keys (baleen currently only uses the latter).

[holic]

Details on cloning with an OAuth token with repo scope are here:
https://github.com/blog/1270-easier-builds-and-deployments-using-git-over-https-and-oauth
https://developer.github.com/guides/managing-deploy-keys/#https-cloning-with-oauth-tokens

[heytrav]

👍 It would be awesome to see if tests are passing before reviewing code.