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).
It's odd that you sometimes do feature testing and sometimes do browser sniffing.   Why not just say:

```
has.add("event-link-onload-api", false);
```

here?  Alternately, do we really need the call to `has.add("event-link-onload-api", true, true, true);` below?

Also, it's inconsistent that you say `navigator` but `global.document`.  Shouldn't the first line also say `global.navigator`?   I assume the `global.` is for the benefit of the builder.