2.0.0 CRAN release

• caretEnsemble now inherits from caretStack
• Removed the optimizers and now use a glm for caretEnsemble (optimizers will be added back ase caret.train models in a future release)
• Cleaned up namespace (all dependencies are explicit imports, rather than implicit imports or dependencies)
• Removed S3 functions that are not really S3 functions (e.g. autoplot and fortify). We will either make those true S3 classes, or inherit from the packages that define them in a future release
• Fixed the build on travis and locally