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 see it as clmath/delite@86766da0aee6e93d0b9b4bd852b33e8d3a66cf91 and added comments there.

BTW, since you guys are both working on this, it probably would have made sense (or maybe still makes sense) to put it in a branch (*not master*) of ibm-js/delite rather than pruzand/delite, so that you can both update it.