Adds Configuration Variable for Icon Font Directory

Hello there, thank you for this library!

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.  Have you guys thought about also configuring the other config variables in the !default syntax so that they can be overridden in a similar manner?

Example from Yeoman Webapp:

```
$icon-font-path: '../styles/fonts'
@import 'flat-ui/sass/flat-ui'
```

Cheers,
nathan