Add explicit text nodes to zoom in/out controls

IE < 9 does not support CSS content properties, so the +/- does not
get rendered. Instead, add the +/- as text nodes when creating the
DOM nodes for the control.

css/ol.css

@@ -219,16 +219,10 @@ a.ol-full-screen-true:after {
 .ol-zoom-in {
   border-radius: 2px 2px 0 0;
 }
-.ol-zoom-in:before {
-  content: "+";
-}
 
 .ol-zoom-out {
   border-radius: 0 0 2px 2px;
 }
-.ol-zoom-out:before {
-  content: "\2212";
-}
 
 .ol-zoomslider {
   position: absolute;