Adds not function for Libsass compatibility

I don’t think ` not ` is safe to use right now due to this missing feature in Libsass https://github.com/sass/libsass/issues/368

I’ve added a `not-()` function that should allow it to return the correct result on both until that issue is closed. I’m not sure if that’s the right syntax necessarily, so I’m happy to make changes based on feedback.
Yeah, it was for Libsass v2.x, but it still seems to work fine without that (at least with Node-sass v0.9.6), so I restored it to the original using `or`. Good catch.