United States' Squadron and Group Codes added

When the United States entered World War II it was decided to adopt the British system of unit identification codes. This was a system where there was a 2 letter/digit code that indicated the unit, and a 3rd letter that designated the particular aircraft in the unit. Of course there were exceptions to this, but probably 99% followed this method. I've now added the Squadron and Group Codes for the units in the 8th Air Force. I'll add the 9th Air Force later.