Using Medea.
Adding medeadown to dependencies in package.json.
Removing leveldown from package.json
Moving registry tests to MemDOWN instead of LevelDOWN.
Adding database cleanup code (db.close()) on SIGINT, SIGTERM.
Moving database cleanup from registries to zetta.js.
Merge pull request #24 from zettajs/medea

Change underlying DB implementation from LevelDB to Medea.