Manage db schema with rubenv/sql-migrate

It's currently hard to make changes to the database schema. Use sql-migrate to make incremental changes.
Stop hard-coding the database name (the default is still "certs" for backward-compatibility)
The `automigrate()` function will automatically run pending migrations.
Use a different migration directory per database driver. This carries a cost of duplication, but is easier than creating migrations which will cleanly execute in both SQLite and MySQL.
Migrations are shipped using the packr utility.