Ch.3-6 Added EightPuzzleState and actions
Ch.3-6 Added check for actions.tail in SimpleProblemSolvingAgent
Removed aima-example module due to circular dependency
Ch.3-6 Fixed errors in Search, Node, and GraphFrontierExpander

Also reduced size of test case in PropositionalParserTest to reduce testing time
Ch.3-6 Added breadthFirstSearchTest. Optimized graph node expander
Ch.3-6 Removed FrontierExpanders, replaced with immutable NodeExpanders