[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 you can pull the `y - safe_sparse_dot(X, coef)` term out of the loop, and I think you can calculate the `loss_derivative` for all active features in one matrix product too, rather than in the loop.