This change changes the behaviour of notification to also send messages directly to users. This requires users in Slack and ReviewBoard to match (documentation added). This provides a nice feed for each user to get updates on only the reviews they care about. The users it notifies are the submitter, any people/group members assigned the to review and any commenters.
Also this adds a header on the payload sent, Content-type: application/json to support the Slack open-source alternative MatterMost. (This follows the slack documentation's request to have this header present)

Tested with MatterMost, which claims 100% api compatibility with Slack.