Splitting off adapters as separate libraries

 - couldn't really come up with a #define method
   that works for both compile and link phase without
   forcing users to set -D flags to linker/compiler (which is 
   easy in Eclipse/impossible in Arduino IDE?) 

 - so resorted to splitting into separate libraries 
   so that if you want to use Two Servo motor bot
   then load both BackSeatDriver and BackSeatDriver_TwoServoAdapter.