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* Changed default behaviour to upload the staging tree instead of the main tree
* Added ``--deploy`` flag to reproduce old bahavior (upload to main tree)
* Added thumbnail generating methods for audio, HTML5, PDF, and ePub nodes.
Set the ``derive_thumbnail=True`` when creating the Node instance, or pass the
command line argument ``--thumbnails`` to generate thumbnails for all nodes.
Note: automatic thumbnail generation will only work if ``thumbnail`` is None.