[TwigBridge] Replace default list style to icon

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Replaces default list style in **bootstrap_3_layout.html.twig** to icon.
Requires Glyphicons from Bootstrap 3.

Before:
![2014-10-07 11-55-50](https://cloud.githubusercontent.com/assets/1592053/4540807/d6f8eb46-4e0b-11e4-82bf-77b56fd1d17c.png)

After:
![2014-10-07 11-55-07](https://cloud.githubusercontent.com/assets/1592053/4540810/e02fe048-4e0b-11e4-9f3b-69902c0ae32b.png)


what do you mean by "does not work" ? This is how they are displayed in the default theme: https://github.com/symfony/symfony/blob/d9131014f599e34e42f751ab546efd2fa2ca3b32/src/Symfony/Bridge/Twig/Resources/views/Form/form_div_layout.html.twig#L292