isObservable

Determines whether a type qualifies as an observable.

An observable must have the following members:

• Event: the type of the events emitted by the observer - this must be an instance of ObservedEvent
• `connect(C, A...)(ref SignalConnection, C callable, A args): Used by observers and operators to get notfied of new events emitted by an observer
• closed: A boolean property that signifies whether the observable has been closed (cannot emit any more events)