Adding extra parameters for querying Cloud Storage keys.
Testing interaction with page token and extra params in iterator.

Also making sure tests pass by using the `upload_` prefix
instead of the `set_contents_` prefix.
Using {} as default in iterator for extra_params and get_query_params.
Updating docstring in Iterator.get_query_params.
Merge pull request #332 from dhermes/add-params-to-key-iterator

Adding extra parameters for querying Cloud Storage keys.