Adding access control at the item model layer and adding PUT to API.

The check for permission of the parent folder is now done at item load.
Fixing up the item view and making the edit work.

This also included some model changes such that access control is checked on
the item to determine which actions to allow. This is done by passing a
callback to getAccessLevel.
Adding a GET /item/<id>/files call and fixing style and files test.

The file download and delete calls were failing in the test because of checking
the folder level which is now done at the item level.