Yaml

Hi,
This is an incomplete yaml vocabulary using the FFI to libYAML.
I suggest we discuss here the features we want to implement before this can be pulled. Also, please review the code and suggest/implement improvements.
- [ ] cross platform test
- [ ] more types (convert to/from factor integer, float, f, etc.)
- [ ] support anchors (for example recursive data structures)
- [ ] do something with tags

We can look at http://pyyaml.org/ to get ideas for a nice API from factor