Add getWatchedRepositories, isWatching, startWatching and stopWatching

Notes:
___
- Added a utility function of sorts here called "hasRepoName". I wasn't sure if you wanted to do this, or to start adding these types of things to a utility module, as it may be useful in the future for other things.
- stopWatching is a DELETE so a successful call returns a 204 according to   [Github's documentation](https://developer.github.com/v3/activity/watching/#delete-a-repository-subscription).

`cb null, s is 200, h`