Move python library files to lib/bup/

Move python library files to lib/bup/

...and update other programs so that they import them correctly from their
new location.

This is necessary so that the bup library files can eventually be installed
somewhere other than wherever the 'bup' executable ends up.  Plus it's
clearer and safer to say 'from bup import options' instead of just 'import
options', in case someone else writes an 'options' module.

I wish I could have named the directory just 'bup', but I can't; there's
already a program with that name.

Also, in the name of sanity, rename memtest.py to 'bup memtest' so that it
can get the new paths automatically.