python tutorial: cleanup imports
improve move_group_interface tutorial

Clarify use of display_publisher.
Fix cartesian path request so it can succeed.
Add path planning with obstacle.
move_group interface tutorial: plan around obstacle

add code to generate a plan while a world box is in the way.
move_group tutorial: fix timing & error checking
cleanup tutorial text (comments)
move_group tutorial: update expected output
Merge pull request #39 from acornacorn/hydro-devel

Update move_group interface tutorials