Pass the document's key to migrations

Release 0.3.3

Improve write and save method docs

Merge pull request #67 from nezuo/preserve-metadata

Update changelog

Preserve metadata for save and close

Merge pull request #66 from nezuo/remove-lock-after-failed-close

Update changelog

Remove lock from document if close fails in specific ways

Merge pull request #65 from nezuo/collection-remove

Update changelog

Add Collection:remove

Merge pull request #64 from nezuo/no-yield-before-callbacks

Fix changelog

Update changelog