Log cv

Continuation of the work by @fabianp and @GaelVaroquaux 

TODO's

I think the some of the private functions in logistic.py can be simplified. In particular, it is my intuition that we could squash some of the functions _logistic* and _logistic_*_intercept without sacrificing a significant amount of performance

sparse matrix support (any help from the people who actually use sparse matrices would be great here)

implement multinomial logistic or raise a meaningful error for multiclass pointing to OvA, OvO meta-estimators

More tests for LogisticCV object (and some are failing)

Document tolerance and stopping criterions