Server-pushed modification notifications

This version extends upon Revision Control 0.1.0 to add support for the server to reach out and notify the client that there is a newer version available. This version requires that the server be updated to enable the sending of notifications when data is updated, but there are no additional requirements apart from the requirements specified in 0.1.0.

The server may choose the most appropriate communication channel to use for notifying a client about a change, such as WebSocket or Push Notification. Specifically, the server is encouraged to reuse any existing real-time communication channel between the client and the server, such as WebSocket or Server-Sent Events.