Cleanup of trailing whitespace.
Display error message when applicable.
Update field name per Genomics API change.
Faciliate usage without forcing package upgrades.

Note that a full-fledged R package is in the works.  In the meantime, refactored the setup function into two so that users need not upgrade R packages in order to get an oauth token.

Explicitly using rjson::toJSON since rjson::toJSON and   jsonlite::toJSON produce different output.
Merge pull request #5 from googlegenomics/variants-fix

Variants fix and refactoring for setup method.