Support for modulo expressions

Complete with test cases just borrowed from [Sass](https://github.com/sass/sass/blob/stable/test/sass/script_test.rb#L436-L440). This allows [Neat](https://github.com/thoughtbot/neat) to compile; the offending line was [here](https://github.com/thoughtbot/neat/blob/master/app/assets/stylesheets/functions/_private.scss#L3) for reference.

Please let me know if the handling of units is appropriate; haven't had carefully reviewed Sass's code since I'm in a hurry trying to get a design up and running fast. It appears just fine to me, though.