[wip] example_idatasetform without tests

This originated from <https://github.com/okfn/ckan/pull/295> but I've submitted example_idatasetform on its own (instead of with the groupform example in the same pr) and without the tests which don't work yet, and I've done a bit more work to tidy up the plugin and update the docs on this branch.

There's still a couple of things I'd like to do here:

- [ ] The plugin still has to import `ckan.lib.navl.validators` and `ckan.logic.converters`, maybe these should be added to the plugins toolkit?
- [ ] It would be nice to have a working example of convert_to/from_extras() as well as tags, maybe another field should be added to this example to use that