#9 helptext and #7 date widget

To support a new key `help_text`, form_snippets have been updated to use CKAN's info form macro to display any plain text with an info icon as grey text next to the input widget following the examples inside `ckan/ckan/templates/macros/form.html`.

A new form snippet `date.html` is a copy of `text.html` with the additional argument `type='date'`. Similarly, we could add types `email` and `url`.

A custom example schema dpaw_dataset.json is included (and can be discarded, or relevant bits could be reworked into e.g. `camel_photos.json`) to demonstrate the use of help_text, the date widget, and also how to override the default dataset (`"dataset_type": "dataset"`).

The README has been updated with instructions for `help_text` and `date.html`.