Restore the preexisting designated initializer contracts of NITitleCellObject and NISubtitleCellObject.
Calls to any initializer other than initWithTitle:image: or initWithTitle:subtitle:image:
used to traverse subclass overrides of those initializers. The designated initializer chain
introduced in commit d8a69614901e95ff9736938a2df6c8829e10e43e makes more intuitive sense but
violates the previous contract provided to all subclass overrides of the previous designated
initializers and potentially introduces a profusion of cell object initialization errors.
Merge pull request #515 from jverkoey/designated_initializer_fix

Restore preexisting designated initializer contracts for cell objects.