Feature #571 - Closing storage objects - Removed closing(), renamed

Feature #571 - Closing storage objects - Removed closing(), renamed
StorageObjectWrapper, added with-support to CloudFilesStorageWrapper

*   Removed custom `closing()` method
*   Removed usage of `closing()` in process_media/__init__.py
*   Renamed StorageObjectWrapper -> CloudFilesStorageObject wrapper
    In my first version of the StorageOjbectWrapper it was located
    inside the CloudFilesStorage object, things have changed since
    then but there has been no renaming, thank you Elrond for the
    good point.
*   CloudFilesStorageObjectWrapper now supports context manager
    methods such as `__enter__()` and `__exit__()` (and `close()`)