Automated Slack Invite

Pivotal Story: https://www.pivotaltracker.com/story/show/76637290

* Added sucker_punch for background job processing
* Added `SlackInviteJob` which uses Slack API to invite user.
* Added `after_create` hook to `User` model to invite new created user to slack

`SLACK_AUTH_TOKEN` env variable is needed for it to work. Contact me on Slack PM for the token.
It is true that MRI doesn't have true multithreading, it uses mutexes to apparently run multiple threads on the same thread, but I dont think we have any other alternative. It is unlikely that we'll be using JRuby/RBX and we can't use worker dynos, so this is the best way to go for I think. Also, it is considerably more efficient than blocking the main thread by running it synchronously.

Not exactly a proper test, but what I did was I enabled invites in the test environment and ran rspec. So whenever a user was created, the invite job would be run. I didn't notice any large time difference, so I think it should be fine on production.