Check for existing files and only upload if different

[GeertvanHorrik]

I've written something similar and needed to use it inside Cake. If this can be used instead of my own project, that would be nice.

Maybe you can (I can help if you want) integrate parts of that code-base where I submit the MD5 hash as well:

https://github.com/CatenaLogic/AzureStorageSync/blob/develop/src/AzureStorageSync/Azure/DifferenceCalculator.cs

Then the sync of large file sets could be faster.