tilde param syntax

[xqms]

While reading http://wiki.ros.org/roslaunch/XML/param again for #40, I discovered the ~private syntax:

<launch>
        <param name="~private" value="test" />

        <group ns="group_ns">
        </group>

        <node name="joy" pkg="joy" type="joy_node">
        </node>
</launch>

This will result in a parameter /joy/private. I'm not aware of anyone using this - but we should probably support it...