Asset manager does not try to load JavaScript map files.

Asset manager looks at items loaded in the asset pipeline and tries to retrieve them from appropriate paths. 
Javascript map files have a type of 'img' which cannot be loaded from the supported image paths.

This PR checks to see if a map file is being requested, and if so, skips trying to load it, moving on to the next file in the assets array.

This fixes issue Frontier-79.

I opted not to try to fix the underlying problem by adding appropriate paths for map files because I don't believe that developers should include minified versions of code when asset manager ultimately minifies them for deployment.