Change not to avoid libsass bug, provide some SassScript to replace missing Ruby functions

There's an open libsass bug that causes `not` to need to be treated as a function. https://github.com/sass/libsass/issues/368 Because Compass uses the not keyword widely, we needed to update those ~130 cases to use `not()`.

Compass also provides a number of Sass extensions in Ruby. Here, we're providing most of the list functions as SassScript variants, allowing the `background` mixin to work.