[WIP] 2750 add docs and examples for idatasetform and igroupform

I've updated example_idatasetform for CKAN 2.0, but not example_igroupform yet.

<del>example_idatasetform is still not working because of bugs in CKAN that break IDatasetForm</del> `example_idatasetform` on this branch is updated for CKAN 2.0 and working, but only tested manually, haven't got the unit tests running yet. `example_igroupform` is not updated for CKAN 2.0 yet.

Also I cannot get it to load the plugin when running the tests.