#7 date and #9 helptext

To address #7, 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`.

Re #9, 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`.

The README has been updated with instructions for `date.html` and `help_text`, a few examples have been added to `camel_photos.json`.
You're right. Should not have included `package_version_validator`. I'll see whether I can whip up a date_validator.