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