libyaml integration

YAML is an ugly mess and the only reasonable C parser I found for it was https://php-yaml.googlecode.com/svn-history/trunk/parse.c

This CFEngine-specific implementation maps YAML into our in-house JSON library, so timestamps and many other advanced YAML features are not supported.  I did the best I could but this could use some review and tough love.  It could also use the same testing as `readjson()`.

The user-facing functions `readyaml()` and `parseyaml()` are provided, with acceptance tests.

Finally, if it's accepted, the function `writeyaml()` (not trivial but doable) will be needed.
Since it depends on a separate library and the rest of ``json.c`` will compile without it, I really would like it to reside in a separate file. But you can call it ``json-yaml.c`` instead to mark its relation to ``json.c``.

Unless you see any technical difficulty in doing it?