Add substitution nesting

Rationale:
 - Needed for supporting eg. comparison of params (see last comment in https://github.com/ros/ros_comm/pull/341#issuecomment-37585632)
 - It enables stuff like eg. &lt;include file="$(find $(arg robot)_description)/robots/upload_$(arg robot).launch" /&gt;.
 - Why shouldn't we have this? :)
I moved '$(find roslaunch' to the line below, ie. from no-op to parse error.

This is because the new parser will throw an "Unbalanced parentheses in substitution args" error. If there's some value in keeping the old behaviour, this can be removed by just removing the two lines at the end of _parse_args(), but I found it made more sense to complain about this probable typo.