Update README

Fix #40, #41 and #42
Improve the graph output
Fix the Git backend when some nodes have children and contents at the same time

We now store the content key in a .contents file at the root of the directory
Update and fix the tests

We need to clean-up the in-memory Git store between the tests. This is a regression introduced by the Contents exposure in the different backends.
Fix the Graph output (again)
Tell the user when the generated dot file is corrupted
Improve debugging
JSON encode/decode server-side exceptions