Moving forward by distance could be made tolerant of obstacles

[jwalthour]

As of 924609f, the PID-based "drive forward N feet" move, upon meeting an obstacle, will continue to drive forward until the wheels slip enough to represent N feet of movement. It would be great if the move would "give up" when the robot is stuck and can no longer roll its wheels forward.

There are a number of ways to detect this:

1. Check for excess current
2. Have a "timeout" where the move automatically gives up after a number of seconds
3. Notice when the rate of change of encoder distance stays below a certain threshold for a certain amount of time