adding the variable $fa-used-icons to reduce CSS file size

with the new variable, a user can limit the icons in the CSS file onyl to those, he really uses. This will significantly reduce the CSS file size from around 20K (compressed) to someting like 5K when only a couple of icons are used.

## Usage ##

The variable is predefined in the `_variables.scss` file:

```scss
$fa-used-icons:       "" !default;
```

The user can define a list with the icons, he want's to use and only those icons will be included into the CSS file:

```scss
$fa-used-icons:       bookmark comment code;
```