Added WebP support

*WebP* — Image format that employs both lossy and lossless compression.

Browser support in a nutshell:
- Firefox 4: Partial
- Chrome 7-8: Partial
- Chrome 9+: Yes
- Opera 12+: Yes
- Opera mobile: 12+: Yes
- Android 4+: Yes

Partial refers to use of .webp files inside a `<video>` tag