Merge pull request #317 from plietar/master

Enable tracing in hooks when DOKKU_TRACE is set.
Split commands into independent hooks.

This prevents commands from hanging when called from ssh.
Also use `pluginhook -s` to run commands.
Add a simple interactive shell to run dokku commands.
Allow the logs' `-t` option to come before or after the app name.

Also fix a problem with detaching from the container.
Add a few utility commands.

A `docker command which (obviously) runs docker, and an `ls`command
which lists the deployed applications.
Prefix all commands in plugins with the plugin's name.

Don't use pluginhook to run commands.
Renamed 00_dokku-standard into core.