Devocarew file icons

Add file icons to the files view:

- update to the latest version (4.1.0) of font awesome
- use `::before` selectors and `content` css to assign icons based on css classes
- `file_item_cell.dart` displays image icons, html/xml icons. For types that will open in ace with syntax highlighting, we use the 'text file' type icon. For files we can open but that will not open with syntax highlighting we use the black file icon.
- some tweaks to position the icons

![screen shot 2014-05-22 at 10 20 57 am](https://cloud.githubusercontent.com/assets/1269969/3063283/2e671f68-e239-11e3-99e0-b80a2c5cf447.png)


@dinhviethoa