- Implemented YAML import/export roundtrip tests.
- Corrected export.yaml.t regarding headers.
- Modified export.yaml, export.csv and export.ical to clear any
context.headers messages so they don't interfere with the output.
- Preliminary export.yaml support. No unit tests yet, and no decision
on including this feature. It may be that libyaml is the right choice,
as an optional dependency.
