Adding extra parameters for querying Cloud Storage keys.

@tseaver This is the PR I [referenced][1].

Will implement your suggestion in a 2nd commit.

Also notice in #319 that there should be an easier way to pass in `extra_params` than to manually create a `_KeyIterator`.

[1]: https://github.com/GoogleCloudPlatform/gcloud-python/pull/319/files#r19765957
OK I changed the expected behavior of the tests and updated in 6851159347f02f42c217b846f7e6bdfb68f6bcbe

PTAL