Issue title: how to integrate with menu stored in database
 Issue body: i have table menu which stores my menu and it is multi level.
Menu
-id
-parent_id
-menu_name
-url

how can i implement this with knpmenu? any hints

 Comments: 
 Comment 0: Look at the NodeInterface: https://github.com/KnpLabs/KnpMenu/blob/master/doc/01-Basic-Menus.markdown#creating-a-menu-from-a-tree-structure

 Comment 1: that does seem to work as expected. should this be closed now?

{'source': 'GithubIssues', 'repo_name_with_owner': 'KnpLabs/KnpMenu', 'issue_id': 9792251, 'repo_stars': 1316.0}