ENH add randomized hyperparameter optimization

doc/modules/classes.rst

doc/modules/grid_search.rst

doc/tutorial/statistical_inference/model_selection.rst

doc/whats_new.rst

examples/grid_search_digits.py

examples/svm/plot_rbf_parameters.py

examples/svm/plot_svm_scale_c.py

sklearn/grid_search.py

sklearn/tests/test_grid_search.py

sklearn/utils/testing.py