Checkbox label

Issue: https://github.com/phiamo/MopaBootstrapBundle/issues/369

I think this should do it pretty simply. Not really much changed, just added a new option to the form extension called widget_checkbox_label and you can give it values 'label', 'widget', or 'both.' If you think it belongs in another extension I can move it - The option also defaults to 'both' so there shouldn't be any BC breaks.