Add ability to disable drag-and-drop sorting
Rename option 'disable_sorting' to 'sortable' (and set default value to true)
Disable sorting by skipping JS calls to nestedSortable

- Adjust mouse styles to only indicate drag-and-drop when the list is
  sortable (e.g. the CSS ".ui-sortable" is present).
- Add specs to verify sorting is disabled.
- Remove unused Javascript.
Allow `:sortable` option to take a proc
Update changelog with `:sortable` option