Bugfix

Custom option handling bug fixed.

    <%= f.input :date_field, as: :date_picker, :label => 'Date field',
                input_html: {data: {date_options: {defaultDate: '2012/12/25'}}} %>

Upgrade

Upgraded the underlying JS library to the current version
Refactored the input templates - contributed by taboularasa (David Elliott)
Corrections in README

Upgrade

Upgraded to version 4.0.0 to be compatible with the underlying Eonasdan's JavaScript library.