ENH: add pre_dispatch parameter to cross_val_score function

This adds a new parameter ``pre_dispatch`` to ``cross_val_score``. The same parameter is used in ``GridSearchCV `` https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/grid_search.py#L524

Tests are passing, docs arre build correctly and I checked for PEP8.

Feel free to disregard this PR if not of any use.