Changes:
- field.py:
  - added SHIP_TYPE
  - added isStackable method (returns True if type allows crates on that field)
- ship.py:
  - added ship's coordinates (size)
- map.py:
  - added pause field
  - added setting ship fields to SHIP_TYPE
- display.py:
  - added pause (press SPACE to toggle on/off)
  - modified ship's drawing

Please modify crane.py so that it passes crates to SHIP_TYPE fields and informs ship about it.