MSIE 10 and 11 support

These changes get us a step closer to cryptocat on Internet Explorer. I'm personally not interested in writing a BHO for IE use in Windows (Just install Chrome!), but in order to facilitate writing a Windows Phone 7/8 app, I would like to see these changes made to cryptocat. This should allow desktop and mobile versions of both MSIE 10 and 11 to execute without javascript errors.

This is somewhat related to https://github.com/cryptocat/cryptocat/issues/208