Add support for MiniBatchNMF to NMF scorer

[mdekstrand]

SciKit-Learn now has a minibatch NMF model that should be faster than then (unusably slow) NMF model. We should use it.

• Update LensKit NMF to take another configuration setting, method, that can be either "minibatch" or "full", and defaults to "full".
• When the method is set to "minibatch", use the minibatch NMF to learn the decomposition. Use the normal NMF when it is set to "full".
• Update tests to exercise both versions.

[ywcharles]

excited to contribute!