Re-factored the way the textures and blocks are being built. It should
be easier to understand and add new exceptions (sorta). Also fixed the
water and lava with other texture packs by putting a static water.png
and lava.png in with the code.
This causes the parent process to re-raise the exception when it
eventually tries to get() the result from the child, instead of hanging
forever on a ctrl-C. It doesn't happen immediately, which is not ideal,
but at least things will eventually exit on ctrl-C now.
