readme updates, texture code updates.

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.

world.py

@@ -74,6 +74,8 @@ def render_chunks_async(chunks, caves, processes):
                 kwds=dict(cave=caves))
         resultsmap[(chunkx, chunky)] = result
 
+    pool.close()
+
     # Stick the pool object in the dict under the key "pool" so it isn't
     # garbage collected (which kills the subprocesses)
     resultsmap['pool'] = pool