[SPARK-2511][MLLIB] add HashingTF and IDF

This is roughly the TF-IDF implementation used in the Databricks Cloud Demo: http://databricks.com/cloud/ .

Both `HashingTF` and `IDF` are implemented as transformers, similar to scikit-learn.
Will the return type of this show up okay in Java? It would be good to add a Java example that uses this just to check that everything works.