[WIP] Strong rules for coordinate descent

Fixes https://github.com/scikit-learn/scikit-learn/issues/229

This implements strong rules for discarding non active features in coordinate descent. I believe this works. However it is really slow. 

TODO:

- [ ] Decide whether to migrate the strong_active_set and check_kkt_conditions to Cython
- [ ] Fix the lazy centering of sparse matrices that gives slightly different results.
I think this has a typo: this should not be running over `active_set`, it should be running over the strong set. (And I'm not sure I follow your reasoning for excluding the ever-active features from the strong set for this purpose.)