Add an extension for notifying Slack.com

Slack.com is a popular business chat service which targets software developers.
One of its neat features is that it integrates with a lot of third-party
services, allowing them to send notifications.

This change adds a new extension which integrates Review Board with Slack,
using their "incoming webhooks" integration. This will notify for all of the
usual events, including links to the affected review requests.

Testing done:
Configured this extension in my local devserver and ran through the motions.

Reviewed at https://reviews.reviewboard.org/r/6024/