Use Korma and define a default db connection

DB connection can be fully customized for every environment by setting
the following environment variables:
  + CLOJUNAUTS_DB_NAME (defaults to "clojunauts")
  + CLOJUNAUTS_DB_USER (defaults to "clojunauts")
  + CLOJUNAUTS_DB_PASSWORD (defaults to "")
  + CLOJUNAUTS_DB_HOST (defaults to "localhost")
  + CLOJUNAUTS_DB_PORT (defaults to 5432)
  + CLOJUNAUTS_DB_POOL (defaults to "true")