Simplify variables

Simplify variables in a few ways:

1.  Standardize on the sklearn Transformer API interface.  This eliminates the need for separate transformers for different data sets.
2.  Use pickle for serialization in ModelDriver.  Since we need to use pickle when working with sklearn models and transformers, just pickle the entire instance instead of using a combined json/pickle format.  This reduces the serialization code to a just a few lines.
3.  Use overloading for ModelDriver - fit/predict functions and take numpy arrays or anything implementing `__getitem__`

Also added a few transformers to inputs with tests and added test coverage missing in ModelDriver (for fit/predict).