Merge pull request #47 from elmer-garduno/master

Send credentials to the workers to use the refreshToken.
Added instructions on how to run PCA on GCE
Merge pull request #48 from elmer-garduno/master

Added instructions on how to run PCA on GCE
Updated README run PCA documentation.
Merge pull request #51 from elmer-garduno/update_readme_run_pca

Updated README run PCA documentation.
Upgraded to v1beta2
Merge pull request #52 from elmer-garduno/upgrade_to_v1beta2

Upgraded to v1beta2
Added FixedBasesPerReference as the default split for VariantsPca

Renamed split methods, added documentation and unit tests.
Added request, variant and reference span counters.
Added benchmark code for SearchVariants
Parallelize reads from the API using Observables.

Use --num-threads to specify the number of read threads per worker.