Add PubChem substructure searching functions

[akokai]

Implemented Fred's original substructure searching code in a modularized way and with detailed logging.

To try it on methylmercury compounds:

$ python metacamel/tryit.py

Before merging this, still need to:

• Prevent URL encoding errors. I think this should be OK now.
• Integrate with the CMGroup class so that we can use Python objects to keep track of multiple CMGs in parallel.
• Write a function to do batch searches on multiple CMGroups.

Then we can move on to making different kinds of structure-based searches.