use sqlite by default when there is no db config

if a user doesnt config anything for the db, create a default.db sqlite file when attempting to access the db object, and then tries to save.

Triaged for Tipsy 2.0 (#57)

Superseded: Tipsy 2.0 defaults to PostgreSQL, not SQLite. SQLite remains supported when you set driver=sqlite explicitly. Closing as superseded by the Postgres-first default.