Add sane gamma ramp interface

[jayschwa]

The GammaRamp structure is currently just a collection of uint pointers, which isn't terribly useful. The gamma ramp functions should probably take and emit arrays or tuples or something.

[jayschwa]

I'm still not sure what to do about this functionality and I don't want to make backward compatibility promises about it. #65 will cut them out and this issue will stick around for adding them back in at some point.