Add xdan-datetimepicker-rails and datetime_picker input

Adds [XDSoft's DateTime picker](http://xdsoft.net/jqplugins/datetimepicker/) as the `combined_date_time_picker` input for forms.

[IRC](https://botbot.me/freenode/activeadmin/msg/12028495/) prompting this PR.

Please try this out and see if there's any tweaks or suggestions to better implement this, or if you notice any bugs.

<!---
@huboard:{"order":3208.0,"milestone_order":3000,"custom_state":""}
-->

Why it is named `CombinedDateTimePickerInput` and not only `DateTimePickerInput`?