robot_simulator: make initial joint state configurable. Fix #73.

This change makes it possible to set the initial joint state to something else
than the default 'all-zeros'. Launch file example:
```
..
<rosparam param="initial_joint_state">[0.3, 0.1, -0.1, 0, 0.9, 1.2]</rosparam>
..
```
The parameter is checked to make sure the correct number of values is provided
(has to be equal to the number of joints).
Merge pull request #74 from gavanderhoorn/issue73_simulator_init_jstate

Fix issue #73: make initial joint state configurable