Add generic bagging algorithm to spark.ml

The Pipelines API will make it easier to create a generic Bagging algorithm which can work with any Classifier or Regressor. Creating this feature will require researching the possible variants and extensions of bagging which we may want to support now and/or in the future, and planning an API which will be properly extensible.

Note: This may interact some with the existing tree ensemble methods, but it should be largely separate since the tree ensemble APIs and implementations are specialized for trees.