Userpackages refactor

This is a reasonably major refactor, but it simplifies importing into VisTrails.  With these changes, you no longer need the userpackages repository from Nikea, nor do you need to manually symlink, as the setup.py of VTTools takes care of all of that for you.  This is robust on windows (as that is my development machine) but has not been tested with Unix platforms.  @tacaswell @giltis can you look at this on linux and @sameera2004 @licode can you look at this on mac?
Not a fan of blindly un-linking/removing files.  Installation should not risk damaging the user system, the standard behavior is to fail if files exists.