The `asset_url` needs to use also `theme_static_dir`

Sorry for this mistake, I was using harcoded paths in my template.

If the destination changes in the output folder (`THEME_STATIC_DIR`) also the URL used for the assets should change.
Merge pull request #108 from graffic/patch-2

The `asset_url` was also hardcoded instead of using `theme_static_dir`