Exit on browsers with no audio support

This patch is a simple bail out of the module if the browser has no Audio support of any kind (e.g. IE 8 and other older browsers). 

When the module loads it initially runs "new Audio()" without checking (around line 134) , what causes older browser to  crash here. My patch adds a simple exit strategy to avoid this. 

However, it leaves "Howl" and "Howler" undefined and that may not be a good thing either, but moves the error handling to the user´s code. 

Cheers,
Michael