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'm unclear what this function does since it has no comments.  What happens if all the paths are in the same layer?   What if some of the paths aren't in any layer?

Finally, why not replace `getLayersToLoad()` with a simpler `getLayer()` method that only takes a single path?  With the current code, you keep splitting and then rejoining the paths.