webp support

~~Please don't pull just yet.~~ Good to merge!

This branch includes support for webp. Currently I haven't got the tests to work at all (I haven't tried) and it's getting way to late for me to manage to do it without sleep.

Theoretically everything in this branch should work but I haven't had a chans to go thru it and test it. But see it as a beta release of webp support.

Happy coding!