Rdiffweb modification for Python 3 support

Rdiffweb running on Python 3

During the holidays, I had planned to use my spare time to work on Rdiffweb. There are a lot of minor issues I wanted to get fixed. I like to improve Rdiffweb. Even more since I'm using it to offer professional backup storage for small-medium business (Minarca).

So, the plan was to fix minor stuff, but I finally end up making a major move to python 3. I knew it was coming since Python 2.7 end of life is planned for 2020 and some Linux distribution are dropping Python 2.7 (Fedora, Ubuntu).

Rdiffweb upcoming version (currently in develop branch) should work on Python 2 or Python 3.

Adding graphs to view backup statistics
D3js for Rdiffweb