Merge branch 'release/0.1.5' into develop
Expect Parentless instead of nil.
Add stub for Parentless.
Make ParentTracker use Parentless.
Use be_a instead of checking class equality.
Forgot .new
Expect Leaf initial parent to be Parentless
More Parentless expectations.
Flesh out bits of Parentless.
Rewrite #url ready for Parentless.
Fix UrlReferenceable specs.
Do something silly and regret committing it.
Make specs pass.
Start Parentless spec.
Create specs for url and child_url.
Specify Parentless#children.
Make it pass.
Specify #add! and #remove!
Specify #parent.
Spec typo