Merge pull request #1 from nopoe/master

Added new ray casting method using Daniel Cohen's voxel traversal algorithm
Merge pull request #2 from nopoe/master

Added multithreading and fixed step aliasing in renderG, now default render algorithm
Optimized worlds, new backgrounds, etc.

Blocks are now bytes!  World size can be much larger now.

Worlds can now be saved and loaded (with GZIP compression because no one
like 1gb world files).

Backgrounds have been updated.  32 new images of a much higher quality
(thanks to sexy drop shadows).  Background is shuffled each time the
program is run.