Put all messages into their own package

[jstnhuang]

E.g., pr2_pbd_msgs

This is probably good practice overall. A specific problem right now is that you can't write nosetests and do imports like from pr2_pbd_interaction.msg import ArmState, because it will be unable to find the msg module while importing pr2_pbd_interaction and it's not aware of the catkin devel directory for some reason. This can be remedied if you only have imports like from pr2_pbd_msgs.msg import ArmState. You can still write normal unittests, but then you can't trigger them automatically with catkin run_tests.