Adds Configuration Variable for Icon Font Directory

Hit a wall when configuring flat-ui using bower and yeoman,
this commit allows the configuration of $icon-font-directory
to be set before import using sass / compass.

Example from Yeoman Webapp:

```
$icon-font-path: '../styles/fonts'
@import 'sass-bootstrap/lib/bootstrap'
@import 'flat-ui/sass/flat-ui'
```