Add a method to show large images in WWT using TOAST

[astrofrog]

I'm not sure yet how one does this within WorldWideTelescope, but we need to have a way for users to provide e.g. FITS files or RGB images with AVM meta-data and easily overlay these in WorldWideTelescope. This includes automatically dealing with images that are >2048 pixels and need to be (I think) converted to TOAST format.

[astrofrog]

I'm going to keep this open as we don't yet deal with large/TOAST images.