Add support for field names when writing to dataset
Add tests for field name write support
Make values() and items() work in the presence of broken soft links
Modify tests for values() and items()
Merge pull request #335 from andrewcollette/field_names_write

Add support for field names when writing to dataset.
Merge pull request #336 from andrewcollette/broken_links

Fix values() and items() when broken soft links are present
registered DictCompat as a collections.MutableMapping