[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.

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.