Add a 'make install' target.

Add a 'make install' target.

Also change main.py to search around in appropriate places for the installed
library files.  By default, if your bup is in /usr/bin/bup, it'll look in
/usr/lib/bup.  (It drops two words off the end of the filename and adds
/lib/bup to the end.)

This also makes the Debian packager at
	http://git.debian.org/collab-maint/bup
actually produce a usable package.