Add the `extend` function to VirtualCollection

So that we can create subclasses, the way we do for 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>

References:
https://github.com/jashkenas/backbone/blob/master/backbone.js#L1655
https://github.com/jashkenas/backbone/blob/master/backbone.js#L1689

https://github.com/masylum/Backbone.Subset/blob/master/backbone.subset.js#L361

Thank you :)