Added ACF Copy Tool
Update acf_gearbox.lua

Add necessary code for double differentials.
Create doublediff.lua
Update acffunctions.lua

Added E2 function for Steer Rate on double differentials.
Merge pull request #82 from Scottyprogrammer/master

Added Double Differential (tank) transmission
Merge pull request #81 from looterz/acfcopy

Added ACF Copy Tool