Added test case for 20news dataset
Merge branch 'master' into DT-sparse
Added 'issparse' attribute to Splitter base class
Implemented initial sparse support on DT
Added sparse toy tests and fixed np.ndarray type
Added sparse support into RandomSplitter
Separating sparse matrices test
Returned nogil to tree_.predict() and fixed raw data access from sparse matrix
Fixed sparse toy dataset
Added sparse toy dataset tests for weighted-classifier and regressor