Initial re-write of buildModels

Major re-factor, trying to remove all the places where we need to set
seeds ourselves.
started index creation function
Working buildModels

x,y call works, formula call does not.  We need to extract y from the
formula so we can build the resample folds.
Setting indexes on trainControl now

We seem to be able to reliably identify the target variable in formula
and x, y train calls and use it to build resampling indexes for
caret::train calls, so the models will all use the same resamples.  No
tests yet
Merge branch 'master' into indexes
updated old tests to work with new API

They all pass, and R CMD CHECK also passes. WOOO!

Also replaces all gaussian process models with sum’s and all random
forests with rpart to speed up testing.  We can change things back once
the package is more stable and we can wait longer between tests.
Added tests for model args and formulas

Check that arguments headed for the model (like trace=FALSE) works for
buildModels and that buildModels can dispatch train models via formulas.
warning if methodList not unique

methodList fits with default params.  No reason to do it twice: this
isn’t a package for bagging
Wrote caretModelSpec

tuneList is to be a list of caretModelSpec’s.
methodList gets converted to caretModelSpec.

We can now fit multiple models with the same method and different
tuning params.
Fixed documentation error
Can now specify methodList and modelList
added continue_on_model_fail option
Added some missing tests packages
Test fixes

for some reason test_that was freaking out with all the errors
caret:train generates.
fixed kernlab depends
Added S3 methods for caretStack

print/plot/summary/dotplot
Fixed data generation script
buildModels => caretList
Add explicit ipred call

So we know it’s being used in the tests.  I think this catches all the
modeling libraries we’re using in tests.

Also added explicit importFrom
Merge pull request #56 from zachmayer/indexes

Major refactor of buildModels