Fall back to Alex instead of Steve based on UUID

When the php script fails to load the skin file, it now correctly falls back to either the Alex or Steve skin depending on the player, instead of always falling back to Steve.

It uses code from [sh4ni/Minecraft-UUIC-Tools](https://github.com/sh4ni/Minecraft-UUIC-Tools), which is licensed as WTFPL, to determine the fallback skin based on the UUID.