Media type only statements now supported

• Now supports media type only statements. (@include mq('print') or @include mq('not' plus (max, 600px)))
• Added the ability to debug mq statements by setting either the local $debug setting or the global $mq-debug setting to true.
• Added a proper testing/demo environment to the repository (check out repo > npm i > gulp --open).
• Added support for not as a short-hand for not screen media types.