joint_trajectory_controller rqt plugin

Not ready for merge yet, depends on ros-controls/ros_control#165.

The code has quite a few inline TODOs, but the GUI is pretty solid.

Initial plugin features:
- Allows to select any _running_ joint trajectory controller from any
  controller manager.

- Two modes:
  - **Monitor** Joint display shows actual positions of controller joints
  - **Control** Joint display sends controller commands

- Max joint speed is read from the URDF, but can be scaled down for safety.

Snapshot of the standalone plugin, as rendered by KDE:
![rqt_jtc](https://cloud.githubusercontent.com/assets/1128028/3303022/81c6fb3c-f63a-11e3-91fb-2a493297a60a.png)

Snapshot of an rqt perspective combining two instances of the plugin with `rqt_plot` and `rviz`. I think this is really cool:

![rqt_perspective](https://cloud.githubusercontent.com/assets/1128028/3303180/37287310-f63c-11e3-8cdc-ebc6839416a5.png)