Update favicon.ico, add favicon.png and make newer browsers use PNG favicon

The existing favicon is cut off and opaque, and only 16x16 px. The newer favicon is a transparent 96x96 PNG file which shows the entire OpenBazaar logo, non-stretched, and is used in all modern browsers. For IE versions less than 11, a 32x32 ICO version of the newer favicon is used instead, and replaces the existing favicon file.