Removed: Item.id_relpath, Document.prefix_relpath
Renamed item, document, and tree methods.

- Renamed Item methods:
    + add_link -> link
    + remove_link -> unlink
    + valid -> validate
- Renamed Document methods:
    + add -> add_item
    + remove -> remove_item
    + valid -> validate
- Renamed Tree methods:
    + new -> new_document
    + add -> add_item
    + remove -> remove_item
    + link -> link_items
    + unlink -> unlink_items
    + edit -> edit_item
    + valid -> validate
Pylint cleanup.
Fixed change in GUI code.
Merge pull request #44 from jacebrowning/descriptive-names

Make Document and Item creation method names more descriptive.