First implementation of BDTs from scikit-learn...

First implementation of BDTs from scikit-learn. The BDT support is added through a BDTModel. The current implementation builds an additive model in a forward stage-wise fashion; it allows for the optimization of arbitrary differentiable loss functions.

In addition DNNPlotHandler is now renamed to ClassificationPlotHandler since both DNNs and BDTs are now handled by this plot handler. The Types for models is now Classification-DNN or Classification-BDT instead of Classifiaction. This requires a change in the config file and the renaming of already produced models (i.e. Classification -> Classification-DNN)

Closes #21 (closed).