Add missing target (Cm3) and switch to find_package script.

[helenol]

This fixes 2 issues I encountered while trying to run the library from the pre-built kinetic version on Ubuntu 16.04.

1. libCm3.so isn't exported/installed properly, so the nodelet was failing to load when trying to launch camera.launch
   a. Fix: export Cm3 as an install target.
2. My pre-installed version of the Spinnaker SDK wasn't being found (or wasn't finding the include directories) when trying to build from source.
   b. Fix: switch to find_package and provide a cmake find script.
   c. Fix: also find the include directories in the package script.

Let me know if you're ok with the cmake script, otherwise I can take it out but definitely Cm3 should be installed as a target.

With these fixes, it works fine for me from source with an self-installed SDK. :)

[mhosmar-cpr]

Hi @helenol !
Sorry i didn't see this earlier. I am just noticing the same issue you did. I am only having one issue with the script. It seems to find the libSpinnaker.so in the install folder if you do two release builds in a row but not the header. I think this can be fixed by just using Spinnaker_FOUND instead of Spinnaker_LIBRARIES. The install directive for the spinnaker library also needs updating so it will still work. Ill merge this then link the change I'm talking about.

[mhosmar-cpr]

Commit 8c46088 shows the changes I made.