[TwigBridge] Added form_twitter_bootstrap_x.y.z_layout.html.twig

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | not yet

Doc:

``` yaml
# config.yml
twig:
    form:
        resources:
            - form_twitter_bootstrap_2.3.x_layout.html.twig
```

or 

```jinja

{% form_theme form 'form_twitter_bootstrap_2.3.x_layout.html.twig' %}

{{ form(form) }}
```
Sorry for my bad English :/

```jinja
{% block button_widget -%}
    {% set attr = attr|merge({class: (attr.class|default('') ~ ' btn')|trim}) %}
    {{- parent() -}}
{%- endblock %}
```