Add support for generic model subscriptions

[Zach Kanzler]

Though DCRF doesn't have a standardized way of subscribing to all updates to any instance in a stream, it does offer support for this type of subscription, handing off implementation responsibility to the developer.

It would be nice if dcrf-client could support these types of arbitrary subscriptions, i.e.

• Send an arbitrary request (e.g. subscribe_all, or whatever action name is chosen)
• Add a dispatch listener, using a selector of the developer's choosing (e.g. {action: 'update', data: {...}})
• Automatically resubscribe upon reconnection