Add example using pFUnit as a submodule.

[vegardjervell]

Adding the minimal example mentioned in #15. Note that the way this is set up, cmake seems to get confused if PFUNIT_DIR is set (as I guess it usually is if you have an installation of pFUnit), so you need to unset PFUNIT_DIR to get it to build. Theres probably a more robust way to handle that in the CMakeLists.txt (perhaps in pFUnit itself?).

[mathomp4]

Hmm. I think what this shows me is I should whip up some simple github tests for this repo. Never realized there was no CI here.

[vegardjervell]

That would probably be very helpful to people like me, the reason I started digging into this originally was exactly that I wanted to set up CI with pFUnit for our repo :)