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;

src/objectliterals.jsdoc

@@ -228,6 +228,8 @@
  * @typedef {Object} olx.control.ZoomOptions
  * @property {number|undefined} duration Animation duration in milliseconds. Default is `250`.
  * @property {string|undefined} className CSS class name. Default is `ol-zoom`.
+ * @property {string|undefined} zoomInLabel Text label to use for the zoom-in button. Default is `+`
+ * @property {string|undefined} zoomOutLabel Text label to use for the zoom-out button. Default is `-`
  * @property {number|undefined} delta The zoom delta applied on each click.
  * @property {Element|undefined} target Target.
  * @todo stability experimental

src/ol/control/zoomcontrol.js

@@ -32,10 +32,14 @@ ol.control.Zoom = function(opt_options) {
 
   var delta = goog.isDef(options.delta) ? options.delta : 1;
 
+  var zoomInLabel = goog.isDef(options.zoomInLabel) ? options.zoomInLabel : '+',
+      zoomOutLabel =
+          goog.isDef(options.zoomOutLabel) ? options.zoomOutLabel : '-';
+
   var inElement = goog.dom.createDom(goog.dom.TagName.A, {
     'href': '#zoomIn',
     'class': className + '-in'
-  });
+  }, zoomInLabel);
   goog.events.listen(inElement, [
     goog.events.EventType.TOUCHEND,
     goog.events.EventType.CLICK
@@ -44,7 +48,7 @@ ol.control.Zoom = function(opt_options) {
   var outElement = goog.dom.createDom(goog.dom.TagName.A, {
     'href': '#zoomOut',
     'class': className + '-out'
-  });
+  }, zoomOutLabel);
   goog.events.listen(outElement, [
     goog.events.EventType.TOUCHEND,
     goog.events.EventType.CLICK