Initial integration with node-db-migrate.

This converts the `CREATE TABLE` code in `lib/mysql.js` to a migration that can be rolled-back. Also added a test to ensure that it works (and which can be used as a model for future migration tests too).

Migrations are automatically run when the app is launched and when the test database is set up at the beginning of each test. The latter could get slow, but there are ways around it when that happens, such as getting a `mysqldump` of the fully migrated state and executing it as a series of SQL statements. But we'll cross that bridge if/when we come to it.

This pull request satisfies part of #525.