implement `.toJSON` to support `JSON.stringify`

[Raynos]

If you implement a .toJSON method on your observable object then it will work with JSON.stringify.

In case you don't know, JSON.stringify will call a .toJSON method if it exists on the object you pass in and then stringify the return value.

Example:

var o = $.observable(someData);
// your toJSON method called $.observable.remove(o) for me.
JSON.stringify(o);

[erosb]

Thanks, I will implement that for sure