Dynamic layers and lazy selection layer creation

With this change, the user provides a filter function instead of
an array of layers. Selection layers are created lazily, and
addition/removal of layers is not handled by the control to give
the user more options, as suggested by @elemoine.

examples/select-features.js

@@ -46,7 +46,9 @@ var vector = new ol.layer.Vector({
   })
 });
 
-var selectControl = new ol.control.Select({layers: [vector]});
+var selectControl = new ol.control.Select({
+  layerFilter: function(layer) { return layer === vector; }
+});
 
 var map = new ol.Map({
   controls: ol.control.defaults().extend([selectControl]),