New restriction analysis library, v2

This supersedes #148.  I created a new branch so that the emboss db doesn't appear in the history (and also to restart from the current master).  Now, importing the library will have the side-effect of dowloading the db.  Note, though, that setup.py may need to be modified so that the db is present before biopython is written to a directory owned by root (probably just an import is enough, but untested).
I also got rid of the custom Seq class, now I just rely on the standard Seq and pass circular as a kwarg.
Finally, I removed the patch for the old restriction library as it doesn't seem to apply cleanly and I have little interest in figuring out why -- but you can still get it from #148 and give it a try.