Minecraft Diff Chaining

This code allows the launcher to reach versions of the minecraft.jar by applying multiple diff files, so you don't need to provide every possible combination of starting and ending version as a separate diff, just enough to make a complete chain between versions.

E.g., instead of having to provide a 1.4->1.1 diff, it could instead go 1.4 -> 1.3.2 -> 1.2.5 -> 1.1.

It might not be usable directly as-is because it requires some new information: a listing of which versions there are diffs between.  For now I put that in a minecraft_diffs.txt file in the 'launcher' directory, but you might prefer to have it represented in a different way.

Assuming you even want the extra hassle, that is.  :)  Thanks for at least looking, though!