Integrated several improvements from google-genomics-utils.

Hi @deflaux, @kwestbrooks this PR contains several implementation improvements from google-genomics-utils.

1) Added Paginator + RetryPolicy and removed the ad-hoc iterators we used before.
 - The number of retries is a configurable and defaults to 3 with --num-retries.
 - Fix #31 

2) Updated to use the accessToken and use GenomicsFactory instead of clientSecrets on the workers.
 - The driver still needs to use  clientSecrets to generate the token and propagate it to the workers.
 - Fix #28 

Please take a look.