Allow users to close their account

This can be merged once we've run the transition in https://github.com/codecation/trailmix/pull/119.

If someone decides they no longer want to use Trailmix, we should make it super easy to close their account.

This adds a "close my account" link in settings.

All the user's information, their entries, and their Stripe subscription are permanently removed.

![close-account](https://cloud.githubusercontent.com/assets/65323/4601501/79c67726-5104-11e4-8cb2-285645bb9f17.gif)


`before_action` for this might be a bit more idiomatic.