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.

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For backwards compatability. There is already a datetimepicker.

On Sat, Nov 1, 2014, 9:43 PM Timo Schilling <notifications@github.com>
wrote:

> In lib/active_admin/inputs/combined_date_time_picker_input.rb:
>
> > @@ -0,0 +1,28 @@
> > +module ActiveAdmin
> > +  module Inputs
> > +    class CombinedDateTimePickerInput < ::Formtastic::Inputs::StringInput
>
> Why it is named CombinedDateTimePickerInput and not only
> DateTimePickerInput?
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/activeadmin/activeadmin/pull/3000/files#r19709171>.
>