AudioFXGlobal & Caching

• added caching, so repeated loadings to the same url result in only one request and deep copies of the cached AudioBuffers for the following loads
• AudioFXGlobal is a new external class that gets created by the first AudioFX instance
• AudioFXGlobal now has a changeVolumeAll() method
• AudioFXGlobal now has a destroyAll() method
• added a starting volume option to the constructor