Integrate luks handling better into pre-existing code structure

Treat luks devices as 'handleable' (see `Device.is_handleable`). Using
`is_handleable()` instead of plain `is_crypto()` prevents the code from
trying to (un-)lock system-internal devices.
Separate logic to find device by path from unmounting
Add optional command line arguments: device-pathes to be mounted

If udiskie is called without positional arguments it will be run as a
daemon as before. With positional arguments, however, it will just mount
the requested devices and then exit.