Rearrangement of Less and JS

* Organized less files into groups by meaning, producing separate css files - do we actually need to include "invoice" css in any of our pages?
* Organized js files, removed plugin js source from app.js and put into corresponding plugins. Updated plugins.
* Updated all example pages to use rearranged css and js.
* Fixed few typos.

Please take note, that iCheck is updated to 2.0 RC and there are few breaking changes:
* iCheck method renamed to icheck
* it is not possible to override icheck "on top" of another one (for different look/theme), You need to remove it first with 'destroy' (see mailbox example)
* it SHOULD now take care of checkboxes got by AJAX requests