added method to allow migration of a large work item in sets.
minor update.
more refactor about partial migration of a book, and correct subtype for sub items with no copies.
added processing of migration request in a thread.
Added migration batch, and refactored using findbugs.
minor update.
updated the README text.
updated README text.
Merge pull request #4 from nla/mig-bk-md

merged.