Updated specs
Cleaned code presentation
Added EMPTY_CELL constant
Added further case scenarios for the AI
Minor refactoring
Updated total_markers method
Added block fork scenario 3
Added test for fork scenario 4
Added fork scenario 4 test
Added method avoid_fork_scenario_4
Added test for fork scenario 5
Added method avoid_fork_scenario_5
Added test for fork scenario 6
Added method avoid_fork_scenario_6
Added test for fork scenario 7
Added method avoid_fork_scenario_7
Updated ai_move method
Added test for fork scenario 8
Added method fork_scenario_8
Added test for fork scenario 9