Basic portable mode

I noticed that Raccoon wants to write to the registry when running in windows, so I implemented this basic portable mode. It works by adding a new Preference implementation (EmptyPreferences) which
does not provide or store any preferences. It is activated by setting the system
property 'raccoon.portable=true'.

This allows you to run Raccoon in portable mode like so:
java "-Draccoon.home=$PWD" "-Draccoon.portable=true" -jar raccoon.jar (sh, ps)
java "-Draccoon.home=%cd%" "-Draccoon.portable=true" -jar raccoon.jar (cmd)

In the future we could also implement a Preference provider that does actually store the preferences into a file in _raccoon.home_. (or rather use an implementation of someone else)