Augment Spark MLlib adapter

[mocobeta]

http://spark.apache.org/docs/1.2.1/mllib-data-types.html

Currently, "vector" format is supported. Need support for other data types.

[mocobeta]

Todos:

• Allow users to specify additional data to be returned with feature vectors. (i.e. field values contained in each original document)
• For LIBSVM (labeled data point) format, labels (integer value) are needed. Allow users to describe Lucene field values => labels mapping or generate implicitly if the mapping is not given. When generate the mapping implicitly, output it to a file for reuse.

[mocobeta]

Initial implementations have been commited. I'll close this issue. We need references/documentations about that...