Database: Use $user instead of $username

```
services:
    database:
        class: Nette\Database\Connection
        arguments: [
            dsn: ...
            user: ...
            password: ...
        ]
```
Creation of Connection by this configuration doesn't work because the parameter is not $user, but $username which results to this Exception.
```
Unable to pass specified arguments to Nette\Database\Connection::__construct
```
It should be consistent with http://api.nette.org/2.1/source-Config.Extensions.NetteExtension.php.html#75.