Added methods to IBattle

Added methods that I find necessary to the IBattle interface:
getColumns()
getRows()
getMap()
newTroop()
newTurn() // Perhaps not needed?
getCurrentTroop()
attackablePosition(Position pos)
BattleScreen changes to iterator and fix import

Fixed the import of position as it has been moved.
Changed the implementation of the iterator from getMap() as the previous
was kind of dumb.