Default image foramt to not-jpg for transparent images

If a layer is marked transparent, we should try to help the user out by making it transparent for them. This patch adds logic in initialize that changes image/jpeg to either image/png, or, if in IE, image/gif.

Includes tests.

For completeness, this is a duplicate of http://trac.openlayers.org/ticket/423

slight rearranging of the logic for this patch, since the two tests were checking for transparency, using that instead.

doh. funky keyboard hit return on accident. wms2 patch moves logic around a little but has the same effect, only one minor change: TRuE now registers as true for the automatic baselayer/overlay determination. If we'll do it for the image format might as well do it for that as well... right? so i changed one test to accomodate that.

other major change here is adding same logic to WMS/Untiled.js

for the love of sanity, we need to make a real, bonified Untiled/Grid distinction. That is on my *very* short list.

I'm okay with this, but would like buy-in from Tim because he expressed concern about too much magic. This magic is the kind that we've had users ask for, and is pretty easy to override, but at the same time, it's not something I want to go ahead with without approval from Tim.

I think the test messages should say

"transparent = TRuE causes non-image/jpeg format"

instead of

"transparent = TRuE causes image/gif format"

or test explicitly for non-jpeg and then make sure the alphaHack logic is followed, but that is just because it is easy to be picky when you're not the one writing tests.

Thanks for applying the magic.

patch modified and applied. r3554