Integrations for Events API and Interactive Messages

[leechou]

The server can add integrations for Events API and Interactive Messages.

[dblock]

We could use something first class, subclassing Event or InteractiveMessage. Right now you'd have to roll your own API handlers.

Examples:

Events: https://github.com/dblock/slack-strava/blob/master/slack-strava/api/endpoints/slack_endpoint.rb#L86
Interactive Messages: https://github.com/dblock/slack-strava/blob/master/slack-strava/api/endpoints/slack_endpoint.rb#L66