HopperChains pass along tuples instead of images.

Instead of passing an image directly, the hoppers and hopperchains
will now pass tuples of length 2.  The first element of the pair is
the numpy array image, and the second is a dictionary of metadata.
Merge pull request #6 from wil-langford/hopperchains_process_tuple_instead_of_image

HopperChains pass along tuples instead of images.