Add github status plugin.

Adds the following commands:

github status = Show latest status update for Github. (will auto-print every 5 minutes when github's status is not 'good'):

    github status: 2013-01-13T07:59:53Z - good

github last message = Show latest manual status update for Github:

    github last message: 2013-01-11T15:02:20Z - good - Everything operating normally.