Replace .ico with .png for WP8 browser support

The Windows Phone 8 browser is unable to display .ico images, so
replacing with .png. Appears no other references to favicon.ico in onyx,
so removing it.
Enyo-DCO-1.1-Signed-off-by: Jeremy Thomas <jeremy.thomas@palm.com>
Merge pull request #138 from germboy/master

Replace .ico with .png for WP8 browser support

Reviewed-By: Ben Combee (ben.combee@palm.com)