Add support for generating signed urls

[jacobwegner]

See https://cloud.google.com/storage/docs/access-control/signed-urls for details.

Prior examples to draw from:

• https://github.com/GoogleCloudPlatform/storage-signedurls-python
• https://github.com/GoogleCloudPlatform/google-cloud-python/pull/56

[jacobwegner]

This will likely end up in a companion library to django-gapc-storage.