A slight file format changed cause by implemented sprites and animations

I implemented sprites, both animated and static, in the memapper codebase. That required some changes on the file format. This document applies from tag sprites-implemented onwards until otherwise mentioned (or src/merpg/IO/out.clj gets undocumented changes).

Surprisingly you can't view a png image and say whether it's certainly a tileset, a sprite or a spritesheet. Thus I had to change the tilesets' filenames to follow format "TILESET - :kwid - Tileset's name.png". Then I implemented saving of the sprites. They (both sprites and spritesheets, which are to be split to a list of frames on load) follow the filename format "SPRITE - :kwid.png".

Sprite metadata is put into the file called sprite-registry. It's a Clojure map, where keys map to the sprite filenames. On this registry is saved everything about the loaded sprites that can be serialized to s-expressions. In other words, everything but the concrete image data. Expected keys of the sprites and spritesheets are found in the /src/merpg/mutable/sprites.clj.

Next, I think, I'm supposed to design how the hell users are supposed to script this engine. Wish me luck, there's some hacking around nrepl and emacs to be expected