Allow to run the job execution command as a user 

## Usage
### Configure the user provider:
``` yaml
akeneo_batch:
    security:
        user_provider: my_user_provider.service.id
```

### Specify the user to run the command as
```shell
$ php app/console akeneo:batch:job --user="admin" job_code
```

## Note
- The `--user` option is only available once the user provider has been configured (and the service exists).
- When running the command as a logged in user, the first line of the batch log file will contain:
```
[2014-07-09 19:38:46] batch.DEBUG: Authenticated as "admin" [] []
```

## TODO
- [ ] Write specs
- [ ] Fix files header
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