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.
The URL is only used once, so is it necessary to make it a constant?