Fix for alignment of stacked checkboxes and radio buttons

Modified layout for checkboxes and radio buttons so that stacked
widgets align with their inline coutnerparts (as well as other
form fields).

Stacked and inline fields doesn not align:

![Before fix: Stacked and inline fields does not align](http://i.imgur.com/pYm3OBg.png)

...but with the changes in this commit, they will:

![After fix: Stacked and inline fields align](http://i.imgur.com/pYm3OBg.png)

    # Fields
    checkbox_inline = forms.TypedChoiceField(
        label=_('Checkbox Inline'),
        widget=forms.CheckboxSelectMultiple,
        choices=((1, 'Inline 1',), (2, 'Inline 2',), (3, 'Inline 3',), (4, 'Inline 4',),)
    )

    checkbox_stacked = forms.TypedChoiceField(
        label=_('Checkbox Stacked'),
        widget=forms.CheckboxSelectMultiple,
        choices=((1, 'Stacked 1',), (2, 'Stacked 2',), (3, 'Stacked 3',), (4, 'Stacked 4',),)
    )

    radio_inline = forms.TypedChoiceField(
        label=_('Radio Inline'),
        widget=forms.RadioSelect,
        choices=((1, 'Inline 1',), (2, 'Inline 2',), (3, 'Inline 3',), (4, 'Inline 4',),)
    )

    radio_stacked = forms.TypedChoiceField(
        label=_('Radio Stacked'),
        widget=forms.RadioSelect,
        choices=((1, 'Stacked 1',), (2, 'Stacked 2',), (3, 'Stacked 3',), (4, 'Stacked 4',),)
    )

  # Layout Helper
    self.helper.layout = Layout(
        InlineCheckboxes('checkbox_inline'),
        'checkbox_stacked',
        InlineRadios('radio_inline'),
        'radio_stacked',
    )

In accordance with the
[specifications](http://getbootstrap.com/css/#forms-controls) in
Bootstrap (see section *Checkboxes and radios*) I have moved the
`.checkbox` class to a parent `div` element for stacked widgets
(only), and kept `.checkbox-inline` for inline elements only.