Bootstrap in a user-writable directory

Follow up of #74.  This change moves the bootstrap directory to XDG data home.  It's dead simple, and won't break anything.  Ecukes tests pass as expected.  It will cause Cask to bootstrap anew, but I can't think of any trouble this might cause.

I'd like to merge this, because bootstrapping at installation as suggested in #74 turns out to be more complicated than I initially thought.  I doubt whether it's actually possible, but even if, it will require a good deal of thought and care, hence much time.  

To fix root installations meanwhile, we should move the bootstrap directory now and publish a new release.  We can still implement bootstrapping at installation later on.