handle default feature names better

[djsutherland]

Currently, if you pass Features instances to train and test, and don't give categories or names for either, the default names will conflict.

Also happens if you pass a list for the training data and a Features instance for the testing data.

[djsutherland]

Probably it would be best to keep a CityHash value or similar of their contents. Possibly also keep the current "name" field for convenience, or maybe just default names to hash values instead of consecutive integers?