• Refactor Document#resolveValue() into YAML.createNode().
• Drop the internal doc reference from collections, and otherwise simplify the Map and Seq objects by separating out the parsing functions -- this should also make it easier to move or copy a collection between documents. Subsequently, directly calling .toString() on a Map or Seq will now get you a JSON representation of the object; for YAML output you'll need to stringify the whole document.
• Bug fixes, including in particular #18 (reported by @zoroaster00).