[WIP] Towards sklearn compatible subset featurizer.

I'm slowly reworking #133 to be more compatible with sklearn.  

@rmcgibbo are there issues with the way we almost always use `X_list` for our estimators while sklearn tends to use `X`?  Or that the first step in our pipeline involves MDTraj objects?  It seems like we might have some cleanup to do for maximum sklearn compatibility.

(Ignore `selector.py` for now, that's just copy pasted from the previous PR and hasn't yet been updated.)