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:

For example, this input:
```erb
<%= f.input :born_on, as: :date %>
```
Will now render something like:
```html
<input id="user_born_on" name="user[born_on]" type="date" />
```

This sort of mapping for datetime fields is even described in the [README](https://github.com/plataformatec/simple_form/blob/master/README.md) already :see_no_evil: 

Classic 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 a date select on the example above, we should do:
```erb
<%= f.input :born_on, as: :date, html5: false %>
```

Closes #844.