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.
- [x] cross platform test
- [x] more types (convert to/from factor integer, float, f, etc.)
- [x] support anchors (for example recursive data structures)
- [x] do something with tags
- [x] investigate test fail on ubuntu (FAIL with libyaml-0-2=0.1.4-2ubuntu0.12.04.1 ; OK with libyaml-0-2=0.1.4-2); EDIT: fixed by libyaml-0-2=0.1.4-2ubuntu0.12.04.2 (https://bugs.launchpad.net/ubuntu/+source/libyaml/+bug/1279805)
- [x] better error reporting
- [x] cleanup/docs

More advanced features
- [ ] output formatting options
- [ ] schema selection (failsafe, json, core, ...)
- [ ] operate on streams instead of strings
- [ ] factor objects: !factor/foo
- [ ] fancy types (timestamp, merge, value)

We can look at http://pyyaml.org/ to get ideas for a nice API from factor