Allow selection of formatter in asset field views listings

The current 1.x-dev branch doesn't allow this and forces you to use the default formatter. The 2.x-dev branch does allow this, which was really handy - so I added this functionality back into the 1.x-dev branch.

If the selected formatter (e.g. an imagecache option) is not valid for the asset being added (e.g. a youtube video) then it will fall back to the default formatter for that type. It also let's you select whether to make it a link back to the node or not (there is probably a neater way to do that, but it will take more refactoring and I think that is best left to another patch).