Dev

This PR is cleaning up documentation at it fixes a few issues. A few things:

-  Makes sure tuneLength is passed properly to `buildModels`
- Uses a weighted standard deviation to calculate prediction errors in `predict.caretEnsemble` based on the weighted disagreement between ensembled models
- Clean up roxygen documentation and prepare for roxygen2, 4.0

Two things to look at in particular -- 
- Do we like the output from `predict`? It is a little unpredictable actually as implemented, because sometimes it puts out a `data.frame` and other times a `list`. I think this is necessary to preserve the weights when they are adjusted for individual observations, but this may need to be better documented. 
- Do we like move to documentation using roxygen?

Also -- the tests are not yet written for the new `predict` method.