shift to object-oriented

This is the first shift to creating an image data object instead of
using a script. It's not quite working yet because python doesn't copy
objects like I thought... need to work this out.
shift to o-o (again)

these should have made it into the previous commit, but didn't due to
technical difficulties
Exposure class working!

I fixed the copy issue with the exposure class. This involved chucking
the image class. The coroutine behavior has also been cleaned up a
little bit (.next() function is embedded in the __init__() routine).
Also, a histogram and image plot have been added.
Merge pull request #1 from kjgregory/master

Exposure class working!