Copter: Tandem Helicopter

The patch is a first stab at adding support for tandem helicopters (by adding a HELI_TANDEM_FRAME), both for longitudinal (like the CH-47 Chinook) as well as transverse (like the V22 Osprey in helicopter mode). This is configurable using the H_TANDEM_MODE parameter.

Yaw is mixed into the CCPM calculations and DCP (differential collective pitch) is applied for pitch (in longitudinal mode) or roll (in transverse mode). What needs to be added is pre-compensation of yaw when applying DCP.

The testing of this patch has been done using a standard 3-servo swash setup, but I assume that it could support 2-servo swashes (like the Rotomast V22 Osprey) with the correct configuration.

Disclaimer #1: This patch is only bench-tested and I'm currently unable to take it to real hover testing for practical reasons. Hence, the purpose of this pull-request is to make collaboration easier in order for others to help out in tweaking this.

Disclaimer #2: First time ArduCopter hacker. Not an aeronautical engineer.