added support for less-middleware

The less files are now compiled on every request when in development mode and once on server start when in production mode. In views, we include the files as `/css/something.css` and less-middleware looks for `something.less` in `assets/less`, compiles it and puts it in `assets/css` where the client (browser) finds and downloads the css file as requested.

Also instead of using complete builds of bootstrap, flat-ui etc, now we include less files, giving us more control over customization etc.

The `assets/css` directory is now in `.gitignore` because all css is compiled from less. Less was chosen instead of sass or stylus because of wider support. Bootstrap is available in both less and sass. Flat-ui is available in less only. And neither are available in stylus.