Added support for yaml-cpp 0.5+.

The new version of yaml-cpp has a new process for loading YAML documents,
a new iterator interface, and no template<T>Node::operator>>(T&). There's no
version information in the headers, so use pkg-config to test for >=0.5.
Removed inclusion of unused YAML headers.

yaml-cpp/node.h doesn't exist in the new version of yaml-cpp.