Implement basic portable mode

This works by adding the 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)
Note that when using the portable mode, the application always runs with default
settings and does not remember your changes. (the archives including login
credentials are still saved properly, however)