Support for the Slack Events API

[pasali]

Botkit currently supports the RTM API in Slack which is based on WebSockets. Because of this we cannot use BotKit with AWS Lambda. Consider adding support for the Event API of Slack.

[peterswimm]

Coming soon, but if anyone wants to submit a PR with it done, we'd review it! ;)

[impactmass]

@pasali I'm building a Slack command (which I think can rely solely on the Events API). I'm thinking in the line of AWS Lambda like you mentioned. Are you working on suggestions to make Botkit work in this way? I'm willing to sync up with you on that.

[pasali]

@impactmass I don't have time and actually in a bit of a hurry so i did a quick search on google for other 3-party libraries.

[colestrode]

I think this can be closed @peterswimm