Updated favicon to be compatible with old versions of IE and also support Retina displays

IE (certainly older versions) only accepts a genuine .ico file for the favicon, so I fixed it adding a 32x32 version at the same time for displays running at double the pixel density.

I wasn't sure of the font that the 'O' is written in but I think I got a pretty good match. If it's not up to scratch please let me know the font and I can adjust it.