Automated tests

[sjakobi]

This package currently has no test-suite. It would be nice to have one so we realize when things break.

[sjakobi]

Since IIRC JSON is a subset of YAML-1.2, maybe we can test that the conversion JSON -> YAML -> JSON roundtrips.

I was also wondering whether the yaml-test executable needs tests of its own: I suspect not, because it's a very thin wrapper around the decodeValue and encodeValue functions.