added more types for "offset" option

Author: kasparsj

README.md

@@ -116,10 +116,10 @@ as well as (overridden) for each individually:
 All options are **optional**.
 
 #### offset
-**Type:** number  
+**Type:** number or string (percentage or viewport units) or callback function  
 **Default:** 0
 
-If you need to start the scene exact pixels from document's beginning or "triggerElement"'s beginning.
+If you need to start the scene after document's beginning or "triggerElement"'s beginning.
 
 #### duration
 **Type:** number or string (percentage or viewport units) or callback function  

jquery.data-parallax.js

@@ -71,7 +71,7 @@
             typeof options.start === "undefined" || (options.triggerElement = options.start);
             typeof options.trigger == "undefined" || (options.triggerHook = options.trigger);
 
-            typeof options.offset === "number" || (options.offset = 0);
+            typeof options.offset != "undefined" || (options.offset = 0);
             typeof options.triggerElement === "undefined" || (options.triggerElement = convertToElement(options.triggerElement));
             typeof options.triggerElement != "undefined" || (options.triggerElement = this[0]);
             optionsArr.push(options);
@@ -291,9 +291,10 @@
         this.$el = $el;
         this.from = options.from;
         this.to = options.to;
-        this.offset = options.offset;
         this.axis = options.axis;
 
+        this.offset = convertOption(options.offset, this.getElementDimension());
+        
         typeof options.triggerHook != "undefined" || (options.triggerHook = "100%");
         this.triggerHook = convertOption(options.triggerHook, options.axis === Scene.AXIS_X ? windowWidth : windowHeight);
         this.triggerElement = convertToElement(options.triggerElement);
@@ -338,14 +339,18 @@
                 };
             }
             else {
-                var maxDuration = this.axis === Scene.AXIS_X ? this.$el.outerWidth() : this.$el.outerHeight(),
-                    durationPx = convertOption(duration, maxDuration);
+                var durationPx = convertOption(duration, this.getElementDimension());
                 validateDurationPx(durationPx);
                 this.duration = function () {
                     return durationPx;
                 };
             }
         },
+        getElementDimension: function() {
+            return this.axis === Scene.AXIS_X ? 
+                this.$el.outerWidth(true) : 
+                this.$el.outerHeight(true);
+        },
         needsUpdate: function() {
             this.updateStart();
             this.updateDuration();