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.
I am not too confident that our ruby version (MRI) will handle threading very well, it may not block the current request but it could block future requests while this is processing. Have you looked into this?