Add missing bind() to map(getTextureURL)

This only worked before because of the 'game' global used for debugging,
which was being accessed as 'this' in getTextureURL. With the addition
of 'use strict', 'this' is now null instead of window in this context.
Add the bind() call so that this is always the registry instance.
Rename main file to registry.js
Add documentation for item textures API
0.5.0