Convert ChemDB from web.py to CherryPy.

Convert ChemDB from web.py to CherryPy.

I like CherryPy's interface better, and web.py depends on CherryPy
anyway.  Time to cut out the middleman.

Also moved to a non-flat directory structure.

This was a major reorganization, so it may have introduced a few bugs.
I'll probably turn up anything major in the next day or two.

I've learned a lot about Python since I wrote the earlier versions of
ChemDB.  The portions that I've rewritten reflect those changes, but a
good of the unchanged code is still a bit awkward.

Still to come:
* Remove metapost dependency by using PGF/TikZ to generate the NFPA
  diamond.