GRUF, layout, LinguaPlone and memory leaks...

This is quite a big pull request, adressing issues we encountered while doing migrations for customer projects.

From the changelog:

- Avoid memory leaks by adding the posibility of batching content exports. [thet] (for more context on that, also see: https://github.com/bluedynamics/zopyx.plone.migration/pull/1 )
- Fix setLayout and reference field importing. [jensens]
- Handle LinguaPlone translations. Note, in cases where Folders and Documents are mixed within a Folder, the - correct order isn't preserved. [jensens]
- Deal with exporting Plone 2.1 GRUF groups. [jensens]