RAII for Exporters

It would be handful to provide [RAII](http://en.wikipedia.org/wiki/Resource_Acquisition_Is_Initialization) for exporters, especially when there is just one item to export.

    with SomeExporter() as exporter:
        exporter.export_item(item)

would be simple and less error-prone than

    exporter = SomeExporter()
    exporter.start_exporting()
    exporter.export_item(item)
    exporter.finish_exporting()