Add initialize function, the Backbone way

The `initialize` function, the Backbone way, in order to do some things along these lines:

<pre>
	var Subset_Collection_Appointment = Backbone.VirtualCollection.extend({
		initialize: function() {
			this.model = this.collection.model;
			this.url = this.collection.url;
		},
		filter_by_doctor: function(doctor) {
			this.updateFilter(function(appointment) {
				return appointment.get('doctor').id == doctor.id;
			});
		}
	});
</pre>

(no need to do `this.model = this.collection.model;` anymore, courtesy @fcsonline)

Thank you :)