Add option to unsubscribe or unfetch from a model or query immediately

[ghost]

This pull request allows a model or query to ignore model.root.unloadDelay when unsubscribing or unfetching.

model.unsubscribe and model.unfetch now have the function signatures:

model.subscribe(items..., callback(err), immediate)
model.unfetch(items..., callback(err), immediate)

• items: [unchanged]
• callback: [unchanged]
• immediate: (boolean) If true, immediately unsubscribe/unfetch the items without a delay (model.root.unloadDelay)

As a result of this change, it's possible to unsubscribe and subsequently resubscribe to the same collection (with different database queries) without waiting for model.root.unloadDelay to time out.

[ghost]

Looks like query.ref() resolves this more elegantly