Add delete_contents flag to delete_database method

Setting the delete_contents parameter to True enables deleting
a BigQuery database even when it contains data.
Fix bug: get_datasets did not return a list

BigQueryClient.get_datasets now returns a list of dataset resources,
as per the doc string.
Add README doc for delete_dataset delete_contents param
Merge pull request #21 from aneilbaboo/master

Fixed a bug and added ability to delete datasets with data