Contact for Slack, the chat application

[Slack](https://slack.com/) is a popular team chat service that offers integration through webhooks. However, God's default webhooks don't work with it (since it requires different field names).

This commit adds a new Contact for Slack, allowing you to specify just an account name and webhook token in order to integrate God with Slack.

It also offers an easier configuration than trying to do it through a webhook, since you can specify the channels, message formats, etc. through the ordinary God contact configuration.

This PR includes unit tests for all newly-added functionality; these tests are passing for me on Ruby 1.9.3 (I can't get God's development dependencies to work for me on 2.0+, since `rcov` fails to build, so I had to test on 1.9.3, but I can't see anything that would break in later versions of Ruby).

Let me know if you have any questions!