With moveit_msgs now a string ID can be used for robots and objects

[JenniferBuehler]

After getting rid of manipulation_msgs/household_msgs (see issue #32 and #40), now string IDs are used instead of int IDs. The code in grasp_planning_graspit_ros/src/GraspItServices.cpp (search for GetModelNameAndType in code) along with the tail of that will need to be updated for this. At the moment, we are still forced to use int IDs in the service requests.

Additionally, the API tutorial needs to be updated (all launch files not using ints any more, remove section "Background info: Why the mix of name and ID?", etc.).

This relates to issue #32.