Add support for the not expression

This PR adds support for the not expression. Now that we're correctly not parsing static declaration values (https://github.com/sass/libsass/pull/679) my previous implementation (https://github.com/sass/libsass/pull/658) is now viable.

Fixes #368. Fixes #660. Specs added sass/sass-spec#14, https://github.com/sass/sass-spec/pull/95, https://github.com/sass/sass-spec/pull/143.

/cc @nsams