WIP: Use grunt for assets, remove django pipeline, remove ruby, use bower for client-side deps…

This PR was created at the [Open edX Con](http://conf.openedx.org) hackathon.

Here’s a list of what it does:

* Use [grunt](http://gruntjs.com/) for client-side assets management
* Use [bower](http://bower.io/) for client-side assets dependencies
* Remove the django pipeline
* Use libsass instead of ruby sass

Cool things:

* This PR allows you to run everything client-side related on the host (and not on the devstack).
* If ran on the host machine, compiles all LMS assets in 1.8 seconds.