Small optic improvements to the module chooser

FontAwesome can now be used as module-icons. (e.g. config.json entry: "icon_fa" : "fa-twitter")
You can find the FontAwesome Icons [here](http://fortawesome.github.io/Font-Awesome/icons/).
Addition: changed the config.json of "twitter"

Transition effects on the module links (ease-out 300ms) 
-> defined in "boxes.css.opacity-trans"

fixed Issue #27 by removing the failed icon with an inline JavaScript code

minor changes:
added breaks to "css/font-awesome.min.css" for more readability 
changed the space between head an list of the module-chooser from "padding" to "margin"
set z-index from inner-site-wrapper to "-1"
delete `"icon_fa" : "fa-twitter"` to test that core.js can handle an invalid path