Add head action server

Related to #39 .
In this PR,
I added the head_action_server only for head_pan with using SingleJointPosition.
As mentioned in #39 the feedback tilt will not useful, so this time I didn't prepare head_tilt.

The head_pan feedback only include the position of pan. 
Because I can't get speed of pan, the velocity variable in feedback message doesn't make sense.
Head pan can't stop the motion, so when preempt is requested the action server will just return.
For parameter, it only has dead_zone.

I tested on real baxter and confirmed it works. I will send the client PR to baxter_example(I already have) when this is merged.