Form Actions Support

This allows you to add buttons in a collection that prints them all together.

The syntax used on the form objects is:
```
  ->add('buttons', 'form_actions', [
      'buttons' => [
          'save' => ['type' => 'submit', 'options' => ['label' => 'button.save']],
          'cancel' => ['type' => 'button', 'options' => ['label' => 'button.cancel']],
      ]
  ]);
```

The offset button extension may need some adjustments to work here.

*Caveat:*
Bootstrap relies on the presence of whitespace between buttons to show them with proper spacing. Since most of the form stuff in symfony is encapsulated by `spaceless` at one point or another, the only way to ensure space between buttons is by using a `&nbsp;`. Which i did in the template.
Another possible solution is to add a custom style that adds the margin, but this breaks the "don't touch bootstrap" way of things and adds custom CSS, probably not a good idea.