Add Slack notifier

Adding a notifier for [Slack][1] based on the code from the Pushover
integration. The Slack integration uses Incoming Webhooks from the [Slack
API][2].

I can extend this integration to add attachments, etc. For example, we can send the log lines, as in the email notifier, along with the notification, or add custom fields. I wanted to get an initial version out there to get feedback as early as possible.

  [1]: http://slack.com
  [2]: https://my.slack.com/services/new/incoming-webhook