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).
didn't we said we don't? In particular because the cssWatchPeriod is now only useful on very few devices and in the long run won't be useful anymore, so might not be worth adding it now?