feature: add onClickHandler only if needed

Changed default value of onClickHandler setting to null and added click
handler to ProgressiveImage element only, if there is an onClickHandler.
This way no unused click handler is added to image.

src/pig.js

@@ -282,8 +282,7 @@
        *
        * @param {string} filename - The filename property of the image.
        */
-       onClickHandler: function(filename) {
-       },
+      onClickHandler: null,
 
       /**
        * Get the minimum required aspect ratio for a valid row of images. The
@@ -864,7 +863,11 @@
     if (!this.element) {
       this.element = document.createElement(this.pig.settings.figureTagName);
       this.element.className = this.classNames.figure;
-      this.element.addEventListener("click", function (){ this.pig.settings.onClickHandler(this.filename); }.bind(this) );
+      if (this.pig.settings.onClickHandler !== null) {
+        this.element.addEventListener('click', function() {
+          this.pig.settings.onClickHandler(this.filename);
+        }.bind(this) );
+      }
       this._updateStyles();
     }