feature request: train null model

[kohleth]

Hi there,
I am aware of the nullModel function.
But it would be good if there is a caret method called "nullModel" which works with the train function.
This is so that 1) the comparison it makes with other real models will be fair because of the resampling scheme, and 2) the syntax for training model stays consistent.

[topepo]

Good point. I'll put one together for the next release.

[topepo]

I just called it "null". Let me know if you have an issues with it.