Implemented multithreading.
The script now checks if a new version is available.

If a new version is available, it downloads it to the jars folder and regenerates the patches.
Merge git://github.com/GUIpsp/MCRewind

Conflicts:
	patchgen.py
Only download the jar once when there's a new version.
major rewrite
- move to the bsdiff4 python extension module
- implement command line arguments
- make future-proof ;)
Huge-ass refactor.

(or a huge ass-refactor...)
Remove unused execute_bsdiff function.

Orochimarufan removed it before, but it must've magically reappeared when I merged his changes with my stashed
changes.
Return 0 when there is nothing to do.

Before it was returning 1, which is wrong because it's not an error when there is no work to be done.