Implement dexterity field data adapter.
Add dexterity namedfile tests.
Make data collector adapters work for dexterity too.
plone.namedfile support should be optional.
Dexterity tests: test without blob files.
Dexterity tests: use a schema for the custom FTI.
Remove zc.relation comment. Currently just not supported.
Tests: use non-blob named file.
Fix test regression in dexterity tests.
Merge pull request #4 from 4teamwork/jone-dexterity