Update 6.0.6

This is a minor update that introduces the start of a major feature for ArmCom2: Steam Achievements. Three achievements have been added so far, and if you consider the theme that I'm using for them, you'll know that eventually there will be 54 in total. Two achievements remain hidden and unattainable for now, and will be revealed when all of the remaining ones have been added to the game.

[h2]Changelog for 6.0.6[/h2]

ADD: M36 Jackson, M24 Chaffee, M26 Pershing (not used yet)
ADD: First three Steam achievements
FIX: Missing firing crewman definition for co-ax MG on Crusader I CS