Buying and selling ships now works properly
A planet with no ships for sale will have the shipyard button deactivated.
Disabled the shipyard button instead of removing it if the planet has no ships for sale
Merge branch 'master' into sam/dev
Added methods to Ship to add weapons, shields, and gadgets, if there's room.
Added methods to add weapons, shields, gadgets to ships.
Started working towards deprecating LastAdventures.swap().