[MRG+1] Sparse bagging tests

Added a test each for BaggingClassifier and BaggingRegressor to train on dense/sparse versions of the same data with various input parameters, and make sure the outputs are equal. Constrained tests to just SVC/SVR models. It should be up to various models' tests to make sure they handle sparse data correctly.  These tests should just make sure BaggingClassifier doesn't throw a wrench in the works along the way.