Bumped Level-js dependency and removed approximateSize patch.
Updated Copyright and added it to all files under lib/
Implement db.approximateSize.
Updated dependencies.
Better level-sublevel support.
Merge branch 'approximate-size' of github.com:RubenVerborgh/levelgraph into RubenVerborgh-approximate-size
Merge branch 'RubenVerborgh-approximate-size' into v0.9-dev