Add Heroku wrapper to manage preview
Put skeleton for PreviewsController
Follow @houndci guidelines
Process previews asynchronously
Move tarball and heroku app name into Card

...as these will be set by the user, somehow.
Let the user choose and see previews
Make `wait_build_completion` private
Extract poll interval for `wait_build_completion` into a kwarg)
Pass entire buid object into `wait_build_completion` instead of just `id` of it
Minor improvements

* Make sure that destroying an app you don't own does not blow up
* Add TODO in code for logging CLT output
Merge pull request #27 from catchup/preview

Preview Feature :tada: