Several Enhancements & Cleanups (ThresholdCurve, Filters, Meta Classifiers)

I did some clean up and additions to cover more Weka features:

* ThresholdCurve for classifiers
* Additional filters of the weka-dev lib
* Meta classifiers

Furthermore I introduced a ClassBuilder for filters, similar to the Classifiers ClassBuilder added in fa8ee8298442562950934cae55a03e365259a894