FIX: Allow as_grey on RGBA images.

Fix bug in PIMS that broke `as_grey` for images with an alpha channel.
Ok I agree that merging it with white is the most sensible way to deal with this. But I don't see though why merging it an arbitrary background color would be a problem, the code below behaves well on my rgba testfile:

    rgb = im[:,:,:3] # im should be normalized between 0 and 1
    a = im[:,:,3]
    bg = (1.0,1.0,1.0) #for white
    bga = empty_like(rgb)
    for i in range(3):
        bga[:,:,i] = bg[i]
    rgb_composite = (rgb*a+(1-a)*bga)

For this application you can just as well calculate the luminosity of the background and alpha-merge after the conversion to greyscale:

    grey = a*grey + (1 - a)*sum(bg * calibration)

For white, `sum(bg * calibration) = 1` and for black, `sum(bg * calibration) = 0`