Make the command-line interface more user friendly

[briprowe]

Currently, we have to invoke pallet directly. This is clumsy, because it requires the user to know pallet commands. We should write our own command-line interface for common use cases.

Some use cases that should be simplified are:

1. start VirtualBox instance
2. stop VirtualBox instance
3. add new VirtualBox image

[briprowe]

Start/stop are done.