PreloadImages method will auto-flatten the input array

Closes #42

jspsych.js

@@ -161,11 +161,15 @@
         //                  argument is the number of images currently loaded.
 
         core.preloadImages = function(images, callback_complete, callback_load){
-          var n_loaded = 0;
-          var loadfn = (typeof callback_load === 'undefined') ? function(){} : callback_load;
-          var finishfn = (typeof callback_complete === 'undefined') ? function(){} : callback_complete;
+
+            // flatten the images array
+            images = flatten(images);
+
+            var n_loaded = 0;
+            var loadfn = (typeof callback_load === 'undefined') ? function(){} : callback_load;
+            var finishfn = (typeof callback_complete === 'undefined') ? function(){} : callback_complete;
 
-          for(var i=0;i<images.length;i++){
+            for(var i=0;i<images.length;i++){
               var img = new Image();
 
               img.onload = function(){
@@ -177,7 +181,7 @@
               };
 
               img.src = images[i];
-          }
+            }
         };
 
         // core.turkInfo gets information relevant to mechanical turk experiments. returns an object
@@ -417,6 +421,19 @@
             }
             return r;
         }
+
+        // private function to flatten nested arrays
+        function flatten(arr, out) {
+            out = (typeof out === 'undefined') ? [] : out;
+            for (var i = 0; i < arr.length; i++) {
+                if (Array.isArray(arr[i])) {
+                    flatten(arr[i], out);
+                } else {
+                    out.push(arr[i]);
+                }
+            }
+            return out;
+        }
 
         return core;
     })();