Added github watch/unwatch

The bot checks all watched items ever 15 minutes. Note that github rate
limits to 60 requests per hour. At some point, we should fix the module
to allow user auth details to be given to circumvent the error rate.

Commands:
- !github watch repo [username] [repo]
    The bot watches the given repository for new commits.
- !github watch issue [username] [repo] [issuenum]
    The bot watches the given issue for status changes or new comments

!github unwatch repo [username] [repo]
!github unwatch issue [username] [repo] [issuenum]

Misc Bugfixes:
- Doesn't show an empty cell on github onURL if there is no description of
  the repo