Refactoring the base and source parts of BigML

hmm, does this really need to inherit from BaseBigML? it looked as if the latter provided connectivity functionality, which is kind of orthogonal, no?  wouldn't it make more sense to make this class contain an instance of that other class? or have a BigMLResource parent class that does contain that instance?

If the answer to all those questions is 'no', we've got a problem with the BigML class, because it derives twice from BigMLBase (and i'm guessing it'll derive from me even more times when we refactor the other resources)...