add undoPlay method to Board
update pom.xml to use Java 7
Use diamond operator.
removed Test.java
add thread names
improve performance of AlphaBetaPlayer

Remove random behavior (temporarily) and used System.nanoTime for
microbenchmarking. Timed getMove method of two AlphaBetaPlayers playing
against each other with a search depth of 10 for 5 games before and
after changes. On average, the newer version took ~80% as long as the
previous version.