New css loader plugin

This is a proposal for a new css loader plugin that would inject plain css files instead of js-wrapped ones as it is done today.
It is based on the XStyle's css loader (https://github.com/kriszyp/xstyle/blob/master/core/load-css.js).
The stylesheets are loaded via injected ```<link>``` tags and the plugin will notify the module when the stylesheets have been loaded. Since not all browsers support the ```onload``` event on ```<link>``` elements (concretely, it's all webkit-based browsers with a webkit build <= 535), it also provides an alternate code path to check when the stylesheet is ready using a timeout...

This css loader pluging also contains the code required by the build system to build css layers (cc @clmath).
I like it, that's definitely an improvement!  A few minor things:

1. You don't need the second `cleanup()` call (in the polling function), so therefore `cleanup()` can just be inlined into the first function.
2. The inner functions do not need to take `link` as a parameter because they get it through the closure.
2. Therefore, the setTimeout call can be simplified to `setTimeout(loadHandler, 25);`