Middleware for LESS CSS

The middleware will compile LESS files on-the-fly using the [`Assetic`](https://github.com/kriswallsmith/assetic) library. It supports minification and caching, also via `Assetic`. I've been using this in my project for a couple weeks.   Inspired by [`less-middleware`](https://github.com/emberfeather/less.js-middleware) from the node world.