Poster Changes

-Impliments subtypes for posters, allowing for multiple "sets" of
posters. (IE for serial_number 2, subtype 0 (Contraband) is the atmosia
declaration, while on subtype 1 (Corporate Approved) it is a poster of
Nanotrasen's logo.)
-Adds 8 of the aformentioned Corporate Approved subtype posters.  (Can
be ordered at cargo at 4 posters for 8 points.)