Adding timestamp to spritemaps filenames

[msinia]

I think it would be useful to add a timestamp to each of the filenames of the generated spritemaps, so as to ensure that they won't be cached.

As things are now we either have to change the filename of the spritemaps manually on the configuration (and we can only do this if there isn't more than one spritemap per stylesheet) each time we generate the spritemap, or change the name of the file after it has been generated and also all the references on the stylesheets.

By adding a timestamp to all spritemaps most of this issues would be solved. I've been looking into it but I'm somewhat at a loss as to where the logic should be added, it seems as if the filenames of the maps are not centralized in one spot and that might make matters more complicated.

[jnordberg]

Perhaps something like allowing date formatting in output_image

spritemapper.output_image = sprite%Y-%m-%d_%H:%i:%s.png