set the X-Powered-By header in HTTP responses

Signed-off-by: Joshua Rubin <jrubin@zvelo.com>
Merge remote-tracking branch 'github/master' into feature/powered_by
prepend X-Powered-By with "hubot/"

Signed-off-by: Joshua Rubin <jrubin@zvelo.com>
Merge pull request #628 from zvelo/feature/powered_by

Set the X-Powered-By Header in HTTP Responses