Add Classification tree widget
Add icon and fix signal types and handlers for classification tree widget
Tree widgets constructs the learner/classifier and sends it
Weights are optional.
Move init and predict to SklFitter base class.
Merge pull request #135 from lanzagar/sklfitter

Move functionality to SklFitter base class.
Fixed incorrectly generated code by cython.
Merge pull request #129 from janezd/classification-tree

Add Classification tree widget
Merge pull request #136 from thocevar/cython

Fixed incorrectly generated code by cython (BasketReader).
Compute examples and class distributions in tree nodes.
Merge pull request #140 from thocevar/tree

Compute examples and class distributions in tree nodes.