Spark pca example

Hi Cassie, sorry for the long PR, several changes got mixed in. I'm outlining them in a few bullet items. I kept the pca-example out and will create a separate PR once this goes through.

- Upgraded to spark 1.0.2 in preparation to add the pca example. This required small code changes mostly on: 
  - ReadsPartitioner.scala
  - build.sbt
  - README.md
- Separated the Conf class into its own file GenomicsConf.scala
- Added the Google_1000_genomes_phase_1 dataset.
- Increased the maxnumber of results per call to 100.
- Abstracted the VariantBuilder into a RowBuilder interface that can be reused by other types of RDDs namely the simplified VariantCallsRDD that goes with the pca-example.

Thanks!