Complete rewrite of backblaze backend

Hi,

I was having issue with the current backblaze backend implementation, so I did a complete rewrite of the backend using the b2 package available with pip and maintained by the backblaze company.

This implementation is faster (big files are uploaded in chunks) and more resilient to internet breakage. And because it's cool, I added upload progression support.

I think there is still work to done to tune the b2 init (max number of chunks to use for uploading big files, ...)

Here is the implementation: https://github.com/Speedy37/docker-images/blob/master/duplicity/alpine/b2backend.py

I don't care about the licence, so consider my code as public code :)

By the way, I'm new to python (first python code ever) and to bazaar VCS, so I guess this is not the best way to contribute to this project.