Add extend function to create sub classes

To be able to create subclasses similar to Backbone Models / Collections / Views:
<pre>
	var Subset_Collection_Appointment = Backbone.VirtualCollection.extend({
		filter_by_doctor: function(doctor) {
			this.updateFilter(function(appointment) {
				return appointment.get('doctor').id == doctor.id;
			});
		}
	});

</pre>
Reference from Backbone.js:

https://github.com/jashkenas/backbone/blob/master/backbone.js#L1655

https://github.com/jashkenas/backbone/blob/master/backbone.js#L1689