Add ZU.dom2text and Zotero.Utilities.Internal.getDOMDocument
* getDomDocument: returns a detached DOMDocument object
* dom2text (TODO): Currently just returns Node.textContent, but is intended to return Zotero-formatted string based on text formatting in the DOM and the Zotero.Item field that the text is meant for
Virtualize data objects
Don't count on 0 being the user libraryID in Zotero.Libraries
Some other minor tweaks
Merge pull request #575 from aurimasv/async_db-av

Async DB tweaks (attempt 2)