Fold generators are now objects.

  Implement KFold class, which can be used as a
generator in much the same way as weighted_k_fold
could before, but carres meta information so that
the same object can be used multiple times.
Fitting of full model in parallel with cvs.

  This enhancement was suggested by Declan
Groves.  Allow the fill model to be fit in
parallel along with the sub models (fit on folds
of the data during cross validation).  This can
potentially cut cross validation model fitting
time in half.