Frontend Improvements

This PR is about the transfer to Bootstrap. The only page that uses bootstrap is the landing_page. Basically I want to have only one base in the future and it will be base_central_boostrap.

There are two versions of the stylesheets: Less and CSS (Generated on my computer with [CodeKit](http://incident57.com/codekit/) because IE isn't that great with less). I haven't looked deeply into how to make the most of LESS with IE so I've just put a if statement in the header.

The structure of the less files is copied from bootstrap (variable,mixins...).

I've also experimented with a new [CSS convention](http://csswizardry.com/2013/01/mindbemding-getting-your-head-round-bem-syntax/). It is used in the style_with_bootstrap.less file. I'm new to this so if you think something should be called another way, feel free to share. But I think it really help to subdivide part of the design when you're developing.

[LINK](http://playground.learningequality.org:51319/)

![Design](http://f.cl.ly/items/0e130u2w1Q3t2d451i45/Screen%20Shot%202013-06-06%20at%2018.21.08.png)

/cc @jamalex @wangguan59 @mtourek