Added support for YAML-CPP 0.5+.

The new yaml-cpp API removes the "node >> outputvar;" operator, and it has a new way of loading documents. There's no version hint in the library's headers, so I'm getting the version number from pkg-config.

`bwi_planning` also directly references yaml-cpp code in `src/libbwi_planning/cost_learner.cpp` but does not depend on it like it should, so that was added. I think it didn't cause a build failure because an upstream library had already linked against it.