[MRG+1] #3364 warm start in random forests

Related to issue #3364.
Could you also make sure that all random_state values from `clf_ws` are identical to the random_state values in `clf_no_ws`? I.e., compare `set([tree.random_state for tree in clf_ws])` with `set([tree.random_state for tree in clf_no_ws])`