Added a test for linking to an item.
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.
Removed extra line breaks.

Fixes #18
Implemented Item link/unlink by Item.
Merge from 'master' branch.
Allowing more item/document passthrough.
Merge pull request #36 from jacebrowning/link-to-item

Support Item linking using Item objects.
Cache clearing decorators aren't shared.
Git attributes.
Merge from 'master' branch.