Issue/76

### Store joint pointers instead of joint names in controlboard

* Added the vector `m_jointPointers` with pointers to each joint on the `ControlBoardDriver.h`
* The `ControlBoardDriver.cpp` is now using the new vector instead of calling the gazebo::physics::Model method `GetJoint()` at each loop
* The already existent vector `m_jointNames` is still in use for other purposes besides getting pointers to the joints themselves
I guess we can get rid of this two commented lines.