FIX: Handle alpha channel in grey conversion

- Alpha channel is handled by compositing image on white background.
Alpha is normalized to its max value in the image, to be able to handle
different datatypes.
- Typo in ValueError

Follow-up on https://github.com/soft-matter/pims/pull/117
I'm not convinced that (0, 0, 0, 0) should be interpreted as white. Looking at [this function in scikit-image](https://github.com/scikit-image/scikit-image/blob/master/skimage/color/colorconv.py#L667), it looks like they just drop the alpha channel, so (0, 0, 0, 0) -> (0, 0, 0). I think the simplest approach is best, lest we create unexpected behavior for some users.