Bootstrap 3 fixes

This PR does 2 things:

1. fixes the radio/checkbox button rendering in BS3 (#549)
2. adds an option to set ````row_wrapper_class```` (defaults to ````form-group````) to make it easier to put fields into rows (#547)

About the latter, I put it into ````HorizontalFormTypeExtension```` because I saw no reason to add another extension for one single option. However I would with agree that it does not belong there completely either, so I will leave it up to you @phiamo. If you would rather have it in it's own extension (or somewhere else), let me know and I'll make the change.