Moving to further late binding of templating.helper.assets on Image filename callback

This allows this bundle to be used from the commandline for processing images
(if not using templates). Additionally, completely ignores the callback if a
path begins with '/', as all {{ asset }} tag calls (which is where this would
come into play) use relative URLs.
Merge pull request #21 from kenshaw/master

Allow the bundle to be used from the command line