Spark pca example

Hi @cassiedoll, @deflaux I added code for computing the genome wide PCA, please have a look and let me know what you think.

Thanks, 

@pgrosu are you talking about the sim-matrix? at that point the emitted values are already counts of the co-occurrences, the alternative is to serialize the RowMatrix object directly (which is itself a distributed matrix), so it can be loaded in the cluster to operate on it later.