Only ignore top-level /static dir
Fix .css files which couldnt post-process with ManifestStaticFilesStorage due to missing image references

Created chooser_stacked-bg.gif as an empty file -- bootstrap_admin had a .css files reference it but didn't provide it
Use admin_static since that apparently is necessary for these admin pages to hook into ManifestStaticFilesStorage properly
Enable ManifestStaticFilesStorage for cache-busting
Merge pull request #81 from thrive-refugee/cache-bust

Make ManifestStaticFilesStorage work