Create BodyActuator

This branch adds BodyActuator to OpenSim and edits Actuator Class for its vector output. 
You should have two constructors. The default that takes no arguments and the second that takes the Body that you want the actuator to be connected to. The name is only convenient for serializing the connection and finding the Body. But if you are using the API and have the body in hand it would be handy to provide it.