use OpenLayers.Control.DrawFeature to work around onselectstart mixup in IE

fix #855
use OpenLayers.Control.DrawFeature instead of OpenLayers.Control.DynamicMeasure when drawing box or circle to work around an event mixup with document.onselectstart between DynamicMeasure and OL Drag handler in IE
yeah, Stephane and I had the same reaction, it's just plain awful