Example idatasetform without tests second try

Alright, this is ready for review and merge into master and 2.0.

This is `example_idatasetform` without tests (so it doesn't break the build on master). This version exercises both convert_to/from_tags and convert_to/from_extras and uses `get_action()` (without putting model or session in the context, relies on the new autopopulating instead, so it doesn't import `ckan.model`) and the new `get_converter()` and `get_validator()`. Also updates the relevant docs.

Once this is merged I'll make another pr to get the tests for the example extension working (currently the tests pass but you have to edit your `test-core.ini` to run them)