update nestedSortable to 2.0alpha version and tree can collapse
add collapsible and start_collapsed config
update README
Added the ability to subscribe handlers to ajax events
Remove group option as Rails 4 removed assets group
Add `#tree?` predicate method

Used to simplify whether or not the sort method is tree or list.
Simplify sort URL path

Use the current resource's `sort` action, which handles nested and
non-nested resources.
Fix assets precompile with config.assets.initialize_on_precompile = true
Merge pull requests #9 from songlipeng2003/master
Merge pull request #11 from ryancheung/master

Conflicts:
	lib/active_admin/sortable/engine.rb
Merge pull request #15 from zorab47/fix-sort-routes-for-nested-resources
Simpler sort path for index_as_block
Custom actions in index_as_sortable
README up
gemspec up
Require jQuery sortable for latest ActiveAdmin

ActiveAdmin's master branch no longer include the entire jQuery library.
The nested sortable JS library depends on jQuery's sortable library.
Refactor options instance variable to a method
Add `roots_collection` option

Allow users to define custom logic to find the roots of the tree to be
rendered, while staying backward compatible. Users may now do:

ActiveAdmin.register TreeNode do
  sortable tree: true,
    roots_collection: proc { current_user.tree_nodes.roots }
end
Update README.md with docs for roots_collection
Merge pull request #20 from zorab47/add-roots-collection-option

Add option to specify :roots_collection proc