BulkDeleter: normalize return keys and support > 10000 objects

This pull request brings back functionality that was removed in 1.9.

1. normalize keys to match the [documentation](https://github.com/rackspace/pyrax/blob/b1525f684df8bbcc00ba27b3c6b374e2c0cde3c6/docs/cloud_files.md#bulk-deletion)
1. Support deleting more than 10000 objects.