Folded the functionality from DBSAXDocument into the DocumentImpl class.
Now DocumentImpl handles both the reads and the writes to the database.
Eventually, it will implment the full DOM Document interface. Eliminated
the DBSAXDocument.java file as it is no longer relevant.
Updated DBSAXHandler, DBSAXNode, and DBWriter to use the new class.
Some methods from DBWriter should probably be moved into DocumentImpl,
such as the "write" and "delete" methods. Need to evaluate the consequences
of making such a change.