Implementation of backprop / autograd for 1x1 tensors

Script being discussed: https://gist.github.com/Nikolaj-K/672... (Variable names are improved in that gist file, compared to the ones in the video. For example, due to a copy-paste error, I have a misleading "Rev" in an Add expressions class name.) There's also some more links given in the doc-string at the top of that gist page.