Create multi-resolution favicon.ico (16x16+32x32+48x48) for prettier bookmarking

The favicons used at:

  http://www.ubuntu.com/sites/all/themes/ubuntu10/favicon.ico
  http://www.canonical.com/sites/all/themes/canonical10/favicon.ico

only contain a 16x16 variant. When bookmarking some operating systems and browsers are now able to make use of the higher-resolution variants if contained within the same .ico (.ico is a container format).

Ideally the favicon used should contain at least 16x16 and 32x32 optimised versions of the Canonical roundel and Ubuntu Circle-of-Friends. It may also be useful to provide 64x64 and 128x128 for future proofing, particularly if the favicons are likely to be widely copied or cached, such as when creating new themes. Information on .ico:

  http://en.wikipedia.org/wiki/ICO_%28file_format%29

Both MS Windows XP and Canonical Unity use 48x48 pixels by default.