Update docs to show that Spark v1.x is now the default.
Add flag for configurable variantSetId.

Also rename all instances of dataset to variantSet.
Merge pull request #5 from deflaux/spark_pca_refactor

Added variantSetId flag
Merge pull request #34 from elmer-garduno/spark_pca_refactor

Spark pca refactor
Fixed line length.