Correctly import image positions in media_gallery

[bjoern-tantau]

fixes #336

[mblarsen]

Hej Bjørn. I tried the PR it looks like a welcome improvement. I noticed that the images comes in different order on my store views.

danish:

english:

So on the last the main images are put after the "black shoe image".

I don't know if this is related to this PR at all.

Please note that in the Magmi import the image, small_image, thumbnail and media_gallery columns for both stores are identical and magmi does seem to add images in a strange order. I don't know if this is something you want to address.

I tried