Add an Installing configuration

[MeFisto94]

Since overlaying catkin workspaces is not really a feasible choice, rosplan_demos should support being the cmake install process instead of relying on devel.

I'm not that familiar with the ros packaging system, but since I built other packages in the same catkin workspace, I had to remove the call to catkin install, because otherwise either the demos or all the other packages weren't available to the environment.
Thus it would be nice if this package would not require to source devel and would work with catkin install as well.

Sorry for this bad issue description, since I am not familiar with the ros terminologies, it's hard to express what I mean, but TLDR is: this repository should work for a workspace that has the catkin install configuration and by sourcing the install file not the devel one.