Face display update

[barbalberto]

This PR introduces some improvement for faceDisplay client & server and a new testCase for display.

• new port for yarp::sig::image: you can remotely set the image you want to be displayed on the robot. Format, must be 80*32 pixel rgb
• vocab interface similar to iCub's faceExpression

• command line interface supported. e.g.

  yarp write ... /robot/faceDisplay/rpc set face hap

• server has 3 selfTest, they can be run with yarpdev --device faceDisplayServer --selfTest N with N equal to 1, 2 or 3