[SPARK-1281] Improve partitioning in ALS

ALS was using HashPartitioner and explicit uses of `%` together.  Further, the naked use of `%` meant that, if the number of partitions corresponded with the stride of arithmetic progressions appearing in user and product ids, users and products could be mapped into buckets in an unfair or unwise way.

This pull request:
1) Makes the Partitioner an instance variable of ALS.
2) Replaces the direct uses of `%` with calls to a Partitioner.
3) Defines an anonymous Partitioner that scrambles the bits of the object's hashCode before reducing to the number of present buckets.

This pull request does not make the partitioner user-configurable.

I'm not all that happy about the way I did (1).  It introduces an icky lifetime issue and dances around it by nulling something.  However, I don't know a better way to make the partitioner visible everywhere it needs to be visible.