Add pytorch classifier support
This PR adds provisional pytorch support via custom classifiers. In order to support this, a few things were modified in some of the other classifiers:
- Modify
SupervisedSklearnClassifier
to make extension easier for model/pipeline definition, add a few helper properties fornum_columns
,num_features
,num_channels
. - Make the num_* properties part of the public API for Keras classifiers as well.
- Register local classifier module when it is loaded so custom classifiers can be pickled.
Implements and closes #110 (closed).