Made !github watch not suck.

Old github watch downloaded the commits for a repo and did a diff.

New github watch actually leverages Github's events API to see all kinds
of social and techical changes to a repository.

Github watch tells you about:
- Pushes
- Comments on commits
- Issues being closed or opened
- Comments on issues
- Pull requests
- Comments on pull requests
- Forks

Quality of Life:
- You can now configure the module to use an existing user's auth information.
  Doing this will increase your Github-checking-interval from 15 minutes to 1
  minute, as Github provides better rate limiting to authed users.

Bug Fixes:
- If an exception is thrown while fetching, an exception handler restarts the
  github fetching loop.

Removed:
- Watching explicit issues.