Adding several simple normalizations.

Added two normalizations:

* Normalization via [Z Score](http://en.wikipedia.org/wiki/Standard_score)
* Target length normalization (e.g., [RPKM](https://liorpachter.wordpress.com/tag/rpkm/))

@tdanford @carlyeks and I discussed the best place for this, and thought that it made the most sense to have it inside of ADAM, as downstream tools like [RNAdam](https://github.com/bigdatagenomics/RNAdam) will depend on it, and the normalizations are useful primitives across many omics algorithms. Also, I would entertain any comments about whether org.bdgenomics.adam.rdd.normalization is the best package for this.
Out of curiosity, Frank, why (here, and above as well) the explicit call to ``cache`` and ``unpersist``?  Given that these RDDs appear to only be used once...?