Parameterized approach to the icons

With the current implementation of the LESS-code, there is no easy and generic way to add an icon to an arbitrary class. For each icon, a special HTML tag must be added to the DOM. 

This pull request adds the possibility to use the icons as a mixin to an arbitrary class:

```less
li.file:before
{
   .icon(@icon-file);
}
```

The original implementation is still intact with this pull request