Should have a limit on how much the PWM values can increase per unit time

[merose]

Currently when going into manual mode or backing up, the PWM value is allowed to increase immediately to the value required by the PID controller. This appears to interfere with the brake mode of the ESC. Instead, the PWM values should be limited in how much they can increase per unit time. The value should be configurable.

[jeremyolsen]

Backing up was handled by #31. Probably can close.