Made Searcher.document(id) return null for deleted documents.

Also updated QueryTool to show a message instead of metadata for deleted
documents.
Fixed IndexStructure for new indices.
Searcher.document() throws exception for non-existing docs.

* Throws an exception if the document doesn't exist or was deleted.
* Added Searcher.maxDoc() to get the highest document number plus one.
* Added Searcher.isDeleted(id) to check if a document id refers to a
deleted document.
* Updated QueryTool accordingly.