initial multiclass support

[zachmayer]

trafficstars

Initial working version of multiclass ensembling. Work to be done:

• [ ] predict.caretList return a matrix of probs, not a vector for the positive class.
• [ ] predict.caretStack return a matrix of probs, not a vector for the positive class.
• [ ] Internally, makePredObsMatrix just uses positive class probs. Make it use ALL class probs so we can do proper ensembling.
• [ ] Fix the test to do a train/test split and make a confusion matrix

[coveralls]

Coverage increased (+32.5%) to 96.934% when pulling dd523fac02d972ce4608e48a4d6e58388b29a07c on multiclass into 27a45556876774946bd82e21d19a01a84aba35f6 on master.

[dmpe]

@zachmayer still would like to see this being merged :) someday

[zachmayer]

Closing in favor of https://github.com/zachmayer/caretEnsemble/pull/260