Fix broken Draft renaming.

Draft.rename was broken by 7c1f04bc38f4fbf9bdd11c9caa0254bdcdbd78b6, and
it is used in the admin server.
Switch Draft.rename to take a site arg, add tests.

The site argument is necessary to avoid relative paths.
Update CHANGELOG.
Merge pull request #31 from aprescott/draft/renames

Fix broken Draft renaming