Implement or remove face detection

[mbforbes]

If you set a gaze goal to FOLLOW_FACE (GazeGoal.msg), then in social_gaze.py there is an error as the initialization of this action client (self.faceClient) is commented out.

We need either:

1. to fully implement this
2. to remove it
3. a comment as to why it is currently disabled (at the least)