First attempt.  Has some caching and changes time/elevation without
flicker, but still has a layer within a layer.  Committing for a
save point if the next changes all go wrong...
Next pass.  Layers change elevation/time on request with no big redraw,
and no left-over tiles.

Also updated WorldWind libraries to latest nightly build, and added
support for more projections.

Still need to add a better cache (currently just using a HashMap)
Configurable image caching using EHCache now working.
Removed GPU caching, added EHCache and got mostly on-the-fly stuff
working.  Note that this seems to have a few issues from within Eclipse
(and possible on Linux in general), but works nicely on the video wall
which is what counts