Replace gitreceive by a git plugin.

This removes the need for two distinct users.
Git pushes and dokku commands are sent using the same user.
Remove "remote:" in the git hook output
Use DOKKU_ROOT instead of HOME.

HOME is not consistent if dokku is ran e.g. as root (for setup).
The DOKKU_ROOT variable will also make it possible to have the repositories stored in an arbitrary location (works-ish right now)
Don't allow the push if building failed.