- fix preloadjs AbstractLoader's getItem() method definitions

- add missing static context property to soundjs WebAudioPlugin
- use the web audio api definitions in soundjs (AudioContext, AudioNode, etc)

preloadjs/preloadjs.d.ts

@@ -41,7 +41,7 @@ declare module createjs {
         // methods
         cancel(): void;
         destroy(): void;
-        getItem(): Object;
+        getItem(value?: string): Object;
         getLoadedItems(): Object[];
         getResult(raw?: boolean): Object;
         getTag(): Object;