Clipped uibutton text fix (#266)

Only disable marking to bounds if both a corner radius and shadow is configured in a style. 
 - Also excluding setting the corner radius on the UILabel if both a shadow and corner radius is set to avoid the issue in this case as well. 

Note: Using maskToBounds = YES, when no shadow is configured helps avoid #267 (but doesn't fix it for the shadow case), hence why both of these approaches are implemented here.