Disable control system on startup

[irwineffect]

We want the control system to not publish any thruster messages until a control system message has come through, because it doesn't make any sense for the control system to do stuff if we haven't given it any commands anyways.

Relevant files: https://github.com/PalouseRobosub/robosub/blob/dev/src/movement/control_system.cpp https://github.com/PalouseRobosub/robosub/blob/dev/src/movement/control_system.hpp https://github.com/PalouseRobosub/robosub/blob/dev/src/movement/control.cpp

Veteran coders: don't do this issue, let a new member tackle it.

Psuedocode:

//startup
enabled = false

//when a control message comes in
enabled = true

//thruster message publishing
if(enabled == true)
{
    //publish thruster message
}

[skallaher]

Completed by #220.