Version 0.26.0

-  Added AdaBoost and KNeighbors classifiers and regressors (finally closing #7).
-  Added support for [kernel approximation samplers](http://scikit-learn.org/stable/modules/kernel_approximation.html).  (Thanks @nineil)
-  All linear models are now supported by `print_model_weights` (issue #119).
-  Added `f1_score_weighted` metric so that weighted F1 will be calculated even for binary classification tasks.
-  Modified `f1_score_micro` and `f1_score_macro` to also always return average for binary classification tasks (instead of previous behavior where only performance on positive class was returned).