Synchronized group switch

[mathias-luedtke]

Stops group joints before switch, switches modes and does not forward commands until doSwitch was called.
closes #132

Should be compatible to ros-controls/ros_control#209

Needs to be tested on multi-joint HW.

[mathias-luedtke]

Tested with Elmo (Gold and SimplIQ) and Schunk.

[shaun-edwards]

@ipa-mdl, the ROS-I policy is to have all PRs reviewed by at least one other developer. This ensures that more than one person understands the ongoing development. It also helps both reviewers and committers grow in the ability.

[mathias-luedtke]

@shaun-edwards: I would really appreciate review and feedback, but for now I seem to be the only ros_canopen developer. Should other ROS-I developers review it?

This PR has been open for more than 3 months.
I took this time to test it with various hardware platforms and set-ups.
It is required for jade and critical for indigo, since ros_canopen would have stopped working properly with the next ros_control release.
After the new release a rebased #153 can be merged as well.

[shaun-edwards]

In order to get a review, you will have to tag someone in the PR (like @shaun-edwards). For now, I would recommend tagging @ipa-mirb (he needs to start participating in code review anyway). The goal would be to have a ready replacement/alternative source to take off maintenance if needed.