Clipped uibutton text fix (#266)

When both a shadow-* and corner-radius is present in a style class, the corner radius is not applied to any UILabel sub-views to avoid 'clipping' (whilst retaining button.label.maskToBounds = NO). 

However if there are no shadow-* properties in the style, this patch sets maskToBounds = YES instead and avoids applying corner-radius on the sub-layers. 

Using maskToBounds = YES in this case avoid #267 (but doesn't fix it for the shadow case), hence why both of these approaches are implemented here.