refactor mod (#22)

doc/gameid.md

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+# GameId API
+
+## minetest versions >= 5.7 
+
+simply returns `minetest.get_game_info().id`  
+
+## minetest versions < 5.7
+
+approximates the gameid value via a hardcoded table of gameid=>modname 
+and then checks via `minetest.get_modpath()`. If it fails, it falls 
+back to using `xcompat_unknown_gameid` as the id. See the chart in the
+readme for which games are supported

doc/materials.md

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+# Materials API
+
+consult `/src/materials/minetest.lua` at this time

doc/sounds.md

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+# Sound API
+
+## Option 1: Agnostically depend
+
+You can do this by using a custom field in your node def instead of the `sounds` key.
+
+```lua
+minetest.register_node(nodename, {
+    ...
+    _sound_def = {
+        key = "",
+        input = {},
+    },
+    ...
+})
+```
+
+where:
+
+* key: string name of the field from the sound api you want to use, for example `node_sound_stone_defaults`
+* input: table input of fields you want passed to the key field, used to override specific sounds.
+
+## Option 2: Hard depend
+
+add this mod to your mod.confs depends and directly call the sound_api as follows
+
+```lua
+minetest.register_node(nodename, {
+    ...
+    sounds = xcompat.sounds.node_sound_stone_defaults(input)
+    ...
+})
+```
+
+* input: optional table to override some or all of returned values

doc/textures.md

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+# Textures API
+
+consult `/src/texture/minetest.lua` at this time