Refactor to be more "chef"y

Opening this for a public discussion of this refactor of the dokku cookbook. This reproduces the dokku installation/bootstrap process in a way that allows the dependencies to be managed by Chef.

This process will need a few slight modifications once a separate dependencies hook is introduced into dokku plugins (see https://github.com/progrium/dokku/issues/234)

This also adds the ability for SSH keys to be automatically added to the gitreceive hook and allow apps to be removed from docker via Chef attributes.

The cookbook is fully tested via ChefSpec. There are a few areas that will benefit from the release of the new matchers in ChefSpec v3.

Future plans:

* LWRPs (will also clean up some of the recipe code) with pre-written ChefSpec v3 matchers
* data bag/encrypted data bag support for app ENVs and SSH keys

This branch should probably be rebased prior to a master merge.