Added support for 13w42a's new map colors

This PR adds support for the new colors found in Minecraft 13w42a. It also extracts the previously hardcoded value for the number of colors to the #define "NUM_COLORS", in case Mojang adds more colors in the future.

I've tested this on Windows, and confirmed it to work for saving, loading, image importing, clipboard, and generation functions. I haven't yet encountered any bugs caused by these changes.

Note that I did not update the world renderer to correctly display new colors - it will continue rendering worlds with the pre-13w42a palette.

It may be worth replacing NUM_COLORS with a variable, and adding a GUI option so users can choose whether to use 13w42a's extended palette. Simply setting this variable back to 56 will revert support to pre-13w42a for all cases.