Add ability to sign URL from App Engine service account

Can use the [App Identity](https://cloud.google.com/appengine/docs/python/appidentity/functions) API:

``` python
from google.appengine.api.app_identity import sign_blob
sign_blob(bytes_to_sign, deadline=deadline_seconds)
```
