JSON Exporter, part deux

Thought I'd share this for a little feedback if you'd care to give it a look @jhass @jaywink

This currently:
- Boots the current export off to Sidekiq
- Uploads a json file through carrierwave
- Sends an email notifying the user when the job's complete, which includes a link to the upload
- Should pass tests, although we'll see when Travis gets its mitts on it.

It doesn't:
- Have a good UI solution yet (you just click on 'download my profile' and it takes you to a blank screen with status 200; we'll have to do a flash notice or something to tell the user what's happening)
- Include any of the heavy lifting associations we outscoped from the last PR
I could probably be convinced this perform_async call could just live in the controller action, but I do like having the queue/perform methods together on the model.