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
Would this be simpler / clearer as:

    result = {self.PAGE_TOKEN: self.next_page_token} if self.next_page_token else {}

which then makes L83 and L87 unnecessary.