Python bindings for mllib

This pull request contains Python bindings for the regression, clustering, classification, and recommendation tools in mllib.

For each 'train' frontend exposed, there is a Scala stub in PythonMLLibAPI.scala and a Python stub in mllib.py.  The Python stub serialises the input RDD and any vector/matrix arguments into a mutually-understood format and calls the Scala stub.  The Scala stub deserialises the RDD and the vector/matrix arguments, calls the appropriate 'train' function, serialises the resulting model, and returns the serialised model.

ALSModel is slightly different since a MatrixFactorizationModel has RDDs inside.  The Scala stub returns a handle to a Scala MatrixFactorizationModel; prediction is done by calling the Scala predict method.

I have tested these bindings on an x86_64 machine running Linux.  There is a risk that these bindings may fail on some choose-your-own-endian platform if Python's endian differs from java.nio.ByteBuffer's idea of the native byte order.