[SPARK-1266] persist factors in implicit ALS

In implicit ALS computation, the user or product factor is used twice in each iteration. Caching can certainly help accelerate the computation. I saw the running time decreased by ~70% for implicit ALS on the movielens data.

I also made the following changes:

1. Change `YtYb` type from `Broadcast[Option[DoubleMatrix]]` to `Option[Broadcast[DoubleMatrix]]`, so we don't need to broadcast None in explicit computation.

2. Mark methods `computeYtY`, `unblockFactors`, `updateBlock`, and `updateFeatures private`. Users do not need those methods.

3. Materialize the final matrix factors before returning the model. It allows us to clean up other cached RDDs before returning the model. I do not have a better solution here, so I use `RDD.count()`.

JIRA: https://spark-project.atlassian.net/browse/SPARK-1266

Author: Xiangrui Meng <meng@databricks.com>

Closes #165 from mengxr/als and squashes the following commits:

c9676a6 [Xiangrui Meng] add a comment about the last products.persist
d3a88aa [Xiangrui Meng] change implicitPrefs match to if ... else ...
63862d6 [Xiangrui Meng] persist factors in implicit ALS