Only the custom icons are (re)styled

From commit b69e4fb309e932bf917cff21461f5698a824aa49:

> 1) Default icons are not modified any more. Before they were modified by the CSS, which made them appear different. Using the new code original icons won't change in any way.
> 
> 2) Original icons (from the image sprites) are displayed using a custom background image, just like before. They are precisely targeted by class name, so that the custom background image doesn't appear on default icons.
> 
> 3) Font Awesome icons are aligned by centering them. This way the browser will take care of it, and it's no longer necessary to manual align 'em. Furthermore these icons are targeted by class name as well, so default icons won't be changed.
> 
> Other changes are:
> 
> - A few original icons are available within Font Awesome as well, They are no longer available as original icon.
> 
> - The icon 'dots' are no longer bigger than default. The Font Awesome icons are a bit smaller (14px font size) to make them fit nicely.
> 
> - CSS and HTML are tidier (in my opinion).