Releasecandidate

Fixes #19 and #13. Almost passes CRAN checks as report by R CMD CHECK, just need to iron out issues with undocumented test datasets in the `test-caretStack` test suite. 

Still need unit tests on the ensemble predictive error creation and to ensure that predictive errors are being calculated correctly. Need to document them better as well. 

Incidentally fixes the issue that the `master` branch does not build currently.