Adding ports

[matthew-parlette]

Ports should be created either on the fly or during a big bang (although with the current setup, on the fly will be easier).

Port actions:

• examine port (see how big it is, what does it have to trade)
• dock
  • look around
  • trade (future feature)

Ports may work like planets do, and maybe they can both inherit from the same class (maybe EntityLandable?)

[matthew-parlette]

The port actions will be handled in #24 , so this issue will just handle the adding of ports to the game.