provide a FindYamlCpp.cmake

[vrabaud]

seems useful after:
ros-perception/image_common#31

(plus, there were already problems between 0.3 and 0.5)

There is an open issue upstream:
https://code.google.com/p/yaml-cpp/issues/detail?id=127

Even though there is some CMake code here:
https://code.google.com/p/yaml-cpp/source/browse/

Should we comply to their notation ? YAML_CPP_INCLUDE_DIR and not YAML_CPP_INCLUDE_DIRS

[vrabaud]

Proof of concept that works for me here: #31

[wjwwood]

Ok, I'll have a look tomorrow. Sorry for the delay, this got lost in my inbox noise.

[wjwwood]

We can have a FindYAMLCpp.cmake, but we should fix the logic so it actually makes sense first.