(API) Retrieve Contacts based on creation or update date

Currently there doesn't appear to be a way to retrieve contacts based on recent changes. One can implement this offline by caching the entire contact list and doing a diff, but that puts unnecessary load on the API server and isn't particular efficient.

To address this, it would be create if we could pass in a creation or update date alog with the retrieval API call, and only get back those that match (if any).