form-horizontal support for Bootstrap 3.x

It seems the form-group markup for horizontal forms [has been changed  in bootstrap 3.x](http://getbootstrap.com/css/#forms-horizontal ). Thus, the old way (#34) doesn't work any more.

[potenza/bootstrap_form](https://github.com/potenza/bootstrap_form) gem solved this by defining default class for `:left` and `:right` column in `#form_for` method. I think we could place it similarly but in `#inputs` method, like:
```rb
= f.inputs(label_class: 'col-sm-2', wrapper_class: 'col-sm-10')
```
and add some wrapper around the `input.control-group`, to make it look like this:
```html
<div class="form-group">
  <label class="col-sm-2">Foo</label>
  <div class="form-wrapper col-sm-10">
    <input type="text" name="foo" class="form-control" />
  </div>
</div>
```

Before this issue gets fixed, we could hack it ourselves with `label_html` and `input_html` options for now but.. there is no `label_html` options in current version of formtastic =( (see justinfrench/formtastic#865)

Some other ideas?