Add ability to specify a WCS when showing an RGB image

When doing `show_rgb`, a `wcs` may be specified that is used for the image so that overlays can be created.

Unfortunately, `mpl`'s `pcolormesh` doesn't seem to allow RGB inputs, so this can only be done with imshow and therefore with no distortion allowed.  This limitation means you can't use this for very large-scale views where projection matters.

I also don't think I've taken proper care of recording layers... once you add this layer, it can't be removed at the moment.  @astrofrog, how should I do that?  Just add a new `'image_layer_#'`?

Here's a somewhat ugly first attempt showing how it can be useful: this is an RGBA image where `A<1`

![sample_rgb_overlay](https://f.cloud.github.com/assets/143715/1715487/b5998590-61b3-11e3-90c0-86d3dbf33361.png)

p.s. sorry about the whitespace reformatting in imshow... I can undo that if it's too annoying