tiny tweak for better type safety

[stefanoslig]

This tweak improves the type safety on the exported api.

There is one more trivial thing that I wanted to mention, not related to the above issue.
In the core.ts file there is an exported interface called Observable. I believe that this interface which is also used in the react.ts file for example should be better to have a different name or to be private because in the future it might lead to misconceptions. The rxjs Observable and the native js Observable(in the future) have a different shape than the one in this library. So, a name like Rval instead of Observable would be more intuitive. Also I would change ObservableValue => Subject and ObservableAdministration => SubjectAdministration. Traditionally, Subjects are "multicast". That means that a subject can have multiple listeners and state like your implementation. On the other hand an Observable is always unicast.

Nevertheless, they are just names.