Observable class fields support

• Adds observable class fields support:

class Person extends ObservableArray{
		greetings = 'Hello';
}

const cherif = new Person();

cherif.on('greetings', function (ev, newVal, oldVal) {
	    // it should be observable, handle change here
});

// Property change trigger the handler
cherif.greetings = 'Hola';

• Observable class fields support documentation
  #78