Require lossless and alpha support for WebP

This is a fix for #211.

Testing for lossless WebP should help to avoid false positives when a browser does not support lossless or alpha WebP. Testing for "lossless" should be enough, because all browsers that support lossless should be able to render alpha too. ([browser support](https://developers.google.com/speed/webp/faq#which_web_browsers_natively_support_webp))

This means that picturefill will __not__ use WebP images in older browsers that have only "lossy" Webp support:
- Google Chrome (desktop) 17-22
- Opera 11.10
- Native web browser, Android 4.0-4.1

Data URI is taken from Modernizr:
https://github.com/Modernizr/Modernizr/blob/master/feature-detects/img/webp-lossless.js