Implement selling fish

[adamgulde]

As part of the "shop" module, you should be able to sell any fish you gained from the fishing module. Thus, you need to implement some sort of inventory for player for fish (at least), and the shop module needs to access it. In addition, the fish you catch from the fishing module need to actually be appended to this inventory.

== Note:
As of writing this, the shop module is not yet implemented! If you want to take a stab at making a framework for it, go for it! Please comment on this issue though.