initial multiclass support

[zachmayer]

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 dd523fa on multiclass into 27a4555 on master.

[dmpe]

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

[zachmayer]

Closing in favor of #260