Ftw-jsondump

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- Test against Plone 5.1.
[mbaechtold]

- Add configuration option ``field_dottednames``
in order to use short fieldnames.
[jone]

- Ignore missing partials when seleting with "only".
[jone]

- Fix error in metadata partial when owner is missing.
[jone]

- Use UUID partial only on IUUIDAware objects.
This allows us to dump the json of the Plone site.
[jone]

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- Change file_callback signature to also include the key used in the dict.
For dexterity content, the key is different than the fieldname because it
is prefixed with the interface dottedname.

- Old: ``file_callback(context, fieldname, data, filename, mimetype, jsondata)``
- New: ``file_callback(context, key, fieldname, data, filename, mimetype, jsondata)``

[jone]

- Dexterity: support exporting RichTextValue objects.
[jone]

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- Initial implementation
[maethu, jone]