Implementation of Bidirectional search available

After a week of crashed machines and painful recovery/upgrading efforts, I finally got around to implementing a version of the Bidrectional search described on page 79 of the text book. This can be found in aima.search.uninformed.BidirectionalSearch. A fairly comprehensive test driver for this can be found in aima.test.search.searches.BidirectionalSearchTest as the implementation turned out to be a little trickier/subtler than originally anticipated.