[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.
Intellij hints many functions under `JavaConverters` are deprecated, so I followed the message and found the real converter. But it seems to be unnecessary. Changed it back to use `JavaConverters`.