Added wildcard * for observing all changes in model

Resolves #5, allows you to listen to all changes in model.

``` javascript
var points = {bob:0,frank:2};
Observable.makeObservable(points);

points.on("*",function(){ /* React to changes in points for any person*/});

points.set("bob",2); // Triggers callback
points.set("frank",3); // Triggers callback
```