GitHub Enterprise v2

This is a rebound from @johnbellone's PR #94. It addresses a few issues in the original PR:

- [x] Uses `/api/v3` as path to the api in the enterprise instances.
- [x] Doesn't assume `https` as a protocol, since some people use `http`.
- [x] Doesn't hardcode `github.com` as a placeholder to find remotes.

I have access to all of the github enterprises, I'm going to test it for a few days to see if everything is correct.