Get and export the current version from package.json
Test for the User-Agent header, including current version
Add a User-Agent header including the current GitHubot version

This may need to be changed in the near term to be unique to the
current install of GitHubot so that the user isn't rate limited in the
same bucket as all other users. per
http://developer.github.com/changes/2012-10-14-rate-limit-changes/
Merge pull request #7 from jhubert/bug/missing-user-agent

Add a User-Agent header to prevent 403 API responses and close #6