Introducing an example to restore files encrypred by a ransomware

[tonyqui]

I'm pushing this changeset with a double intent.
1 - It shows how to interact with cursors and how to perform some simple actions with files on dropbox (e.g. reading revisions, restore/delete files).
2 - In the unlucky event your dropbox has been compromised by a ransom, you can restore the files (as long as you have available space) using python.

Concerning the second point since I haven't found anything written in python, here's my contribution to python community.

[greg-db]

Thanks!

[rogebrd]

Hello, is this something you are still interested in working on? If so I will be the new reviewer

[tonyqui]

Hello rogebrd, if you believe it could be helpful for the community, please review and merge this PR so that the community can benefit from it.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
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