Fixes for IE:

 - session storage using opener property works differently in IE, as it actually has "opener"
   but gets an exception accessing anything inside it (Not Permitted).
 - on Windows Server, the Audio() constructor throws Not Available.

These changes fix issues running Guacamole client on a locked down IE browser, which is commonly found on Windows 2K8 server.  On such a configuration, Guacamole displays a blank screen and no error message.  I found and resolved these issues on a local build.