[Command] Add create/drop database commands

The PR adds three new commands which allows to create/drop database:

 * `fridge:database:create`: Creates a database.
 * `fridge:database:drop`: Drops a database.
 * `fridge:database:drop-and-create`: Drops & creates a database (if the database does not exist, the command will only
    create it).

Each command uses by default the configured `default_connection`. If you want to use an other one, you can also specify the connection name as argument:

```
$ php app/console fridge:database:create --connection=name
```