<transmission> tags for gazebo_ros_control

I changed a lot here and I'm open to further modification/undoing some of it if necessary...

- Default namespace for ros_control stuff is simply /MYROBOT
- robot_description namespace currently not name spaced because of the effort based controller issue
- Added a default controlPeriod of 0.001 if not specified
- Removed ``<joint>`` tags in favor of ``<transmission><joint/><actuator/></tranmission>`` tags (see below)
- robot_hw_sim_->initSim() no longer accepts a vector of joint names, but rather a vector of 'JointData' structs that also includes the associated hardware interface. This is to be expanded or maybe changed in the future.

Per the way the PR2 did it, and Adolpho's comments, I propose ros_control use something like this:
```
<transmission name="trans1" type="transmission_interface/SimpleTransmission">
  <joint name="joint1"/>
  <actuator name="motor1" type="EffortJointInterface" />
  <mechanicalReduction>1</mechanicalReduction>
</transmission>
```
For now I don't think we should specify a hardware interface for joints, that is too complicated for version one. See the README for more info on how gazebo_ros_control uses the transmission tags.