ENH: add subok flag to stride_tricks (and thus broadcast_arrays)

As is, `np.broadcast_arrays` does not preserve subclasses, even though for the shape manipulation subclasses should not be a problem. This PR adds a `subok` option that allows one to change that, but is `False` by default, so that existing code will continue to behave as is.
Oh, actually if you really hate DummyArray, you can also use the `np.ndarray` class constructor. You still will have to manually call finalize, etc. since that does not allow to set a parent.