Update icon guidelines

Closes #27
Drop Gimp .xcf file; too many sizes to bother
Ignore .DS_Store files from OS X
Re-export favicons with border

Closes #26
Update iOS Web Clip icons to not have a border

Counteracts automatic rounding of corners.

Closes #25
Drop dead icons
Drop outdated iOS Web Clip icon
Use new favicons and iOS Web Clip icons