Refactor out a method to allow me to unmount the filesystem independendly

I found it useful to ensure that the filesystem is unmounted before starting to mount a new one, e.g. if the process crashed before or was stopped while debugging in Eclipse. Then the mount may have not be removed and thus mounting again would fail.

By having a separate unmount() method, I can do a try-unmount in my startup code and thus avoid this problem nicely!

The change does not change the existing code, but simply refactors out a new public method unmount() which can be used from the outside.