Collective-contentexport

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- Add whitelist (only export fields in the whitelist)
[pbauer]

- Document extending and overriding the export.
[pbauer]

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- Sort fieldnames in blacklist by alphabet.
[pbauer]

- Add ability to provide additional export-methods for arbitrary data by
extending ADDITIONAL_MAPPING.
[pbauer]

- Add tests
[pbauer]

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- Localize datetime
[pbauer]

- Prevent uneven dimension of data-dict
[pbauer]

- Fix blacklist
[pbauer]

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- Get content from all languages.
[pbauer]

- Add export for multiple images and files related with RelationList.
[pbauer]

- Allow choosing blacklisted fields from the fields of the selected type.
[pbauer]

- Use http://docs.python-tablib.org for most exports.
[pbauer]

- Add German translations.
[pbauer]

- Moved initial code from client-project to github.
[pbauer]