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.
Merge pull request #172 from robmiller/feature/contacts/slack

Contact for Slack, the chat application