Improvements to AdjacentList.pm:

Improvements to AdjacentList.pm:
	Fixed a typo (ley for key)
	Changed "attach" to "add". May be controversial.
	Made the "parents" relationship *not* cascade deletes :-)
	Added a "descendents" method which just does a horrendous recursion to do
		what you think it does.
	Added four deletion strategies. Would like to improve this in future by
		e.g. overriding delete()

And tests for delete/descendents bits.