Support for Form Actions (grouped buttons)

This adds support for printing all buttons in a single row, as bootstrap expects them to be (as mentioned in #114).

![screen shot 2014-02-05 at 18 09 30](https://f.cloud.github.com/assets/94331/2089536/59ac6fac-8e88-11e3-82e8-84821ac4d2ed.png)

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']],
      ]
  ]);
```

*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.
I'll let @florianeckerstorfer weigh in on this.

To Do:
- [x] Add Custom Type for Button Groups
- [x] Add proper rendering blocks and handle proper off-setting 
- [x] Add constraint to only allow buttons in the group
- [x] Tests?
- [x] Brush off code with phpdocs and etc.