Updated empty_assets, metadata also counts now.

Assets were flagged empty when it had no valid mediafiles. This included
all assets that had metadata but no mediafiles. This is changed to not
have dissapearing assets in search queries with hide_empty_assets flag.
Merge branch 'master' into feature/empty_assets
Change git:// to https:// for submodules to fix issues with http/https proxies
Merge pull request #122 from arjenk/feature/empty_assets

Feature/empty assets