Implement GitHub.API.Repository module

[begedin]

Problem

#1107 will allow us to implement a GitHub.API.Repositories module with, initially, an issues(GithubRepo.t) function which fetches all issues for a github repository.

Considering how we want to deal with our syncing, we may want the function return a stream instead of a loaded list.

This is blocked until we actually implement #1107

[joshsmith]

This is no longer blocked by #1107 because it's done in #1113