translate advanceTransition to advance() and output command.
remove baseUri from the Node
append to comment token to the tree.
Emit Doctype token.
Emit endTagToken.
Implement CleanUp and test comment.
Fix compilation error on removing baseUri.
Add guava support for Immutable collections for concurrency programming
(thread safety) and implement parseFragment
Remove OutputSettings from Document because it is mutable object
associate to the usage of Document object.
Clean up a few TODO task and implement EmitTagCommand.