Support MySQL, PostgreSQL and Migrations

This pr is rework https://github.com/drone/drone/pull/426 without gorm

## Features
1. Support PostgreSQL, SQLite, MySQL
2. Migrations via ORM
3. Test data renamed to fixtures and use Meddler
4. Use DRONE_DRIVER and DRONE_DATASOURCE instead driver/source flags
```sh
DRONE_DRIVER="sqlite3" DRONE_DATASOURCE="/home/bugagazavr/drone.sqlite" make run
DRONE_DRIVER="postgres" DRONE_DATASOURCE="user=postgres host=127.0.0.1 port=5432 dbname=drone sslmode=disable" make run
DRONE_DRIVER="mysql" DRONE_DATASOURCE="root@tcp(127.0.0.1:3306)/drone" make run
```