refactoring of resource class including a py3k fix

tastypie now has a lot of methods to prevent things being hardcoded inside the code. Since MongoDBResource is intended to be used with other projects I removed DB related stuff inside the code. Those things can easily be handled inside the implemented resource.

With Python 2, it's possible to use map() instead of a list whereas this is not possible with Python 3. Hence I added wrapped that line with a list() call. In overall this class works perfectly with py3k atm.