Added support for storing Sessions in MongoDB

Added MongoStore to web.session to support storing sessions in a MongoDB collection. Added appropriate doctests as well (as long as you have MongoDB running and pymongo installed things should run fine). Modified Session class to support user-defined session id creation and validation in order to take advantage of MongoDB's ObjectId object, but should be generic enough to be useful for other stores as well. Also made 'cosmetic' changes in a few places in the two files I was working in.

Tried to follow your style conventions. Hopefully I did alright.