Create BSStrategy.java

I separated methods containing move selection strategies and put them in their own class, thus simplifying the AIPlayer class.

getStreakDirection(prevMove, prevMove2) determines which path the last 2 hits fall onto. This method allows the continueStreak() method to continue traversing this same path for the next move.

continueStreak(prevMove, board, direction) selects the next move based off of the current streak by continuing in the same direction.