Added a test for linking to an item.
Sorting IDs to remove filesystem dependency.

Added back in deleted test.
Added a doctest for zero-digit documents.

Zero digits == never append zeros.
Labeled all integration tests.

New unit tests are required to get full coverage. The tutorial should
also be updated.
Added unit tests to cover the doorstop.core.tree.
Full coverage with unit tests.
Added tree publishing to the tutorial.
'make doorstop' requires 'env'.
Merge pull request #46 from jacebrowning/integration-test-refactoring

Refactor tests to clearly distinguish between unit and integration.
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.
Merge branch 'master' into reorder