Better sync API

[shashi]

right now we do on(identity, observable) to get observable to start sending updates to Julia.

I bet it's easy to make this asynchronous -- one should be able to add the handler after a widget has been displayed -- this would allow InteractNext to not send stuff to Julia by default allowing it to be used entirely client-sided.

[shashi]

Maybe this will work:

1. All observables in a Scope sync automatically when you call on(f, obs) for the first time. Except:

2. An observable can be marked as private using either @private scope["name"] = Observable() syntax or `private(scope, "name1", "name2", ...).

Marking as private means that the observable only exists in Julia. You can access it via Scope["name"] -- setting and getting will work on Julia, but not on JS unless the same object is used as a public observable in another scope. What this allows is to tack on "derived observables" to a Scope. (e.g. ob2 here can be exposed at scope["julia_value"] and WebIO will not try to copy it to the client). Private observables may deal with values that cannot be serialized to JSON (such as Julia functions see JuliaGizmos/InteractNext.jl#3).