Add option to enable logger and define logger class

Fixes #118

Add the option to disable the logger and optionally specify the logger class.

The logger is enabled by default, but you can disable it per client by adding the `enable_logger` option:

    fos_elastica:
        clients:
            default: { host: localhost, port: 9200, enable_logger : false}

You can also override the logger class by specifying a service id in the `logger_class` option:

    fos_elastica:
        clients:
            default: { host: localhost, port: 9200, logger_class: my.logger.service.id }

This might not be the most elegant solution, but I'm open for ideas if somebody has a different implementation.

@TODO: documentation