Added incoming web hooks support

Documentation will come later, sorry!

Basically, incoming post data should be passed to a new instance of
SlackIncoming. In laravel, that would look like `new
ConnorVG\Slack\SlackIncoming(Input::all())`.

New utilities added to Slack, you can do $slackInstance->utilities().
This allows you to do common things whilst having it all cached (manual
removal of caches is supported, incase you need  it). This allows you
to do multiple things without constantly querying the API.

All .list commands are supported here, that’s it. Seem so I only added
the cacheables for now.

You can also do simple auth tests and use the auth test to get certain
datas (such as team_id, team, user_id, user of the api key owner).