Improvement: better handle renaming of files and directory

[mitija]

The current behaviour when I rename a directory is to delete all existing files and then redownload all the files in that directory, even if none of them have been modified. Clearly this could be improved

I suppose this could be handled by maintaining a hash of all files...

This is just a suggestion :-)

[MichaelAquilina]

This is something I eventually want to get working but unfortunately is slightly complicated due to the way amazon calculates its E-Tag values.

Duplicate of #53