Support to date, datetime and time inputs.

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, by explicitly passing the `html5` option.

For example, this input:
```erb
<%= f.input :born_on, as: :date, html5: true %>
```
Will render something like:
```html
<input id="user_born_on" name="user[born_on]" type="date" />
```
Classic datetime selects are still present for cases when the `html5` option is not passed, in order to keep backwards compatibility.

Closes #844.