Support for Issue and Pull Request Comments

Hello Jeff! I noticed on the README that there was currently no support for adding comments to a Trello Card from the comment web hook on GitHub.

This pull request adds that support. I'm not sure if you're aware, but all GitHub pull requests have an attached GitHub issue, even if the repository has issues turned off. This implies that when a comment on a pull request is created it fires the `issue_comment` web hook (there is no `pull_request_comment` web hook). The logic in this pull will check to see if the comment was actually created on a Pull Request or an Issue and find the appropriate Trello Card to act upon.

Here's a sample of what it looks like on Trello:

![Screen Shot 2013-03-11 at 1 34 10 PM](https://f.cloud.github.com/assets/225005/242506/baa02fd8-8a15-11e2-89ef-c35fe489d2e4.png)