[wip] Add ckanext/example_igroupform example extension

This is the `example_igroupform` extension from <https://github.com/okfn/ckan/pull/295>, but without the tests and the `example_idatasetform` that were mixed in together in that pull request.

- [ ] This needs to be updated to work with CKAN 2.0 before it can be merged.
- [ ] Docs for IGroupForm need to be updated (or written, if missing)
- [ ] Note that there were some CKAN core fixes related to IGroupForm on < <https://github.com/okfn/ckan/pull/295>, those may still be needed again on this branch.
- [ ] It probably won't be possible to add working tests for example_igroupform on this branch due to as-yet-unresolved issues with dynamically changing ckan's config and plugins. If so, this can be merged without tests but after merging a new issue should be made to add the tests when we can.