COPYING: Added GPLv2 license text
sparse: add SparseBundle class and convert FUSE handler to use it

By implementing SparseBundle as a RawIOBase class, we can leverage
it as a "regular" python file. That makes the FUSE implementation
that much simpler.
sparse: Move FUSE implementation to a separate new class

Now that the SparseBundle can be treated as a regular file,
the FUSE implementation can use *any* file. So we move it
to a SingleFileFS implementation. It doesn't make a lot of
sense by itself, yet, but eventually we'll want to move it out of the
way.
singlefilefs: remove stub routines
sparse: added proper argument handling
tmfs: initial python implementation