NVidia is a little picker with local worker sizes and does not allow
for None as value.

* Add a test suite.
* Fix numerous `get_info` bugs. (reports by David Garcia and the test suite)
* Add :meth:`pyopencl.ImageFormat.__repr__`.
* Add :meth:`pyopencl.addressing_mode.to_string` and colleagues.
* The `pitch` arguments to·
  :func:`pyopencl.create_image_2d`,
  :func:`pyopencl.create_image_3d`,
  :func:`pyopencl.enqueue_read_image`, and
  :func:`pyopencl.enqueue_write_image`
  are now defaulted to zero. The argument order of `enqueue_{read,write}_image`
  has changed for this reason.
* :meth:`pyopencl.MemoryObject.get_image_info` now actually exists.
* Add :meth:`pyopencl.MemoryObject.image`.