v0.0.4 - 2013/11/04:    Implemented 'add', 'remove', 'link', and 'unlink' commands.
                        Added basic document tree validation.

v0.0.3 - 2013/10/17:    Added the initial Document class.
                        Items can now be ordered by 'level' in a Document.
                        Initial tutorial created.

v0.0.2 - 2013/09/25:    Changed 'doorstop init' to 'doorstop new'.
                        Added the initial Item class.
                        Added stubs for the Document class.

v0.0.1 - 2013/09/11:    Initial release of Doorstop.
