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 can be extended 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