Use WebP binaries directly

Don't merge until https://github.com/1000ch/node-cwebp-bin is moved to imagemin org and I've pushed some fixes for that.

Fixes #2.
Because it tends be weird sometimes. Feels like execFile is more reliable. Since this is used in most of the imagemin plugins I don't want to break things. The spawn issue could have something to with us using streams in imagemin before though.