Allow RGB cubes to be specified from arbitrary data, not just .png's and .jpg's

@astrofrog - this is primarily to allow testing for #112, but will be useful in its own right.

In order to allow RGB data cubes to be passed in, right now the headers need to be hacked to ignore `NAXIS3` and some other parameters.  This is far from ideal.  I think the `rgb_cube` flag is necessary, though.

Right now, the sole test passes, but I'm not sure if I have the orientation correct - will have to check that later.