Refactor resource tree toolbars and icon nodes to use Font Awesome

Removes all the pixel icons in favor of Font Awesome throughout the entire tree for Resources and Elements, and cleans up the styling associated with those. In addition, makes all icons easily swappable by editing the SCSS build files.

![editing__home_on_the_range___modx_revolution](https://f.cloud.github.com/assets/234237/1274739/7f1fbe62-2dc9-11e3-9be7-ec310ad63989.png)