[SPARK-4081] [mllib] VectorIndexer

**Ready for review!**

Since the original PR, I moved the code to the spark.ml API and renamed this to VectorIndexer.

This introduces a VectorIndexer class which does the following:
* VectorIndexer.fit(): collect statistics about how many values each feature in a dataset (RDD[Vector]) can take (limited by maxCategories)
  * Feature which exceed maxCategories are declared continuous, and the Model will treat them as such.
* VectorIndexerModel.transform(): Convert categorical feature values to corresponding 0-based indices

Design notes:
* This maintains sparsity in vectors by ensuring that categorical feature value 0.0 gets index 0.
* This does not yet support transforming data with new (unknown) categorical feature values.  That can be added later.
* This is necessary for DecisionTree and tree ensembles.

Reviewers: Please check my use of metadata and my unit tests for it; I'm not sure if I covered everything in the tests.

Other notes:
* This also adds a public toMetadata method to AttributeGroup (for simpler construction of metadata).

CC: mengxr

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #3000 from jkbradley/indexer and squashes the following commits:

5956d91 [Joseph K. Bradley] minor cleanups
f5c57a8 [Joseph K. Bradley] added Java test suite
643b444 [Joseph K. Bradley] removed FeatureTests
02236c3 [Joseph K. Bradley] Updated VectorIndexer, ready for PR
286d221 [Joseph K. Bradley] Reworked DatasetIndexer for spark.ml API, and renamed it to VectorIndexer
12e6cf2 [Joseph K. Bradley] Merge remote-tracking branch 'upstream/master' into indexer
6d8f3f1 [Joseph K. Bradley] Added partly done DatasetIndexer to spark.ml
6a2f553 [Joseph K. Bradley] Updated TODO for allowUnknownCategories
3f041f8 [Joseph K. Bradley] Final cleanups for DatasetIndexer
038b9e3 [Joseph K. Bradley] DatasetIndexer now maintains sparsity in SparseVector
3a4a0bd [Joseph K. Bradley] Added another test for DatasetIndexer
2006923 [Joseph K. Bradley] DatasetIndexer now passes tests
f409987 [Joseph K. Bradley] partly done with DatasetIndexerSuite
5e7c874 [Joseph K. Bradley] working on DatasetIndexer