I changed `DateTimeInput` to support the new HTML5 form helpers that
comes with Rails 4 (`date_input`, `datetime_input` and `time_input`)
instead of rendering datetime selects:
For example, this input:
```erb
<%= f.input :born_on, as: :date %>
```
Will render:
```html
<input id="user_born_on" name="user[born_on]" type="date" />
```
Datetime selects are still present, but only when HTML5 compatibility
is disabled. It's also possible to render selects even with HTML5 on,
by setting the `html5` option to false.
If we wanted to use an old date select on the example above, we should do:
```erb
<%= f.input :born_on, as: :date, html5: false %>
```
Closes#844.
Rails 4 changes the `date.order` I18n setting to return strings, which
resulted in a missed lookup in POSITION, and a value like `date_i`
instead of `date_1i` for the `for` attribute on `label` elements.
Point the label for datetime inputs at the first select. The date order can be
overridden by i18n and on a per-input basis. Use the specified order to
discover the first select.
Rafael Mendonça França