Added functions, ready for publish on npm

Hey man, just proposing some changes here to get rem into functions, make it publishable on npm, and also some extra mixin stuff, namely:

Media query methods support a 'legacy' argument which tells it to render in @ie8mode: true stylesheets.
.legacy mixin for rules intended for sub ie8 only
.modern mixin for rules intended for above ie8 browsers

I removed the @base argument for mq's, instead making it a global variable: @mediaQueryEmBase, as I don't see a particular reason to have different em calcs for these queries within a single stylesheet, and less pattern matching requires functions to be of a specific arity (number of args)