Added code to allow setup via URL rather than port, ip, or server

Db.connect allows url parameter for connection to mongo as documented at www.mongodb.org/display/DOCS/Connections
This is the style used in heroku examples for their mongoHQ add-on.  
So now you can setup the session store with new MongoStore({url: MONGOHQ_URL})

native-parser:false is the default so I removed that.

I also made some changes to simplify/clarify and make JSLint happy.