BabuDB/Java:

 * bugfix in checkpointer; writing two subsequent checkpoints w/o an intermediate update no longer causes an error message
 * reverted version number to 0.5.1
 * updated changelog

git-svn-id: https://babudb.googlecode.com/svn/trunk@498 d4c7dd96-b7b2-11dd-893d-d98698f66bbf