Allow hubot to react on events

Events might get triggered by scripts.

For example: You have a script, which can trigger and event that some one committed to a repo. This script can be used by anyone, but the reaction should be different by every company. So it's possible to trigger a build, or just post a message in chat.

As a plus with the same event triggered by different scripts, so it wouldn't make a difference if you commit into github or just another (maybe SVN) repo. But other scripts can rely on that 'commit-received' command and act the same way.