Store default value

[curran]

It would be nice if a reactive-property would keep track of its default value. It would simplify the implementation of exposed properties in reactive-model. This should be simple to add, and would not add too much overhead. It might look something like this:

var a = ReactiveProperty(3);
a(5);
a.default(); // Returns 3
a(); // Returns 5
a(a.default()); // Reset value to default

[curran]

One step closer to datavis-tech/reactive-model#31

[curran]

Enables datavis-tech/reactive-model#29