Drop excessive data in test files

[bevzzz]

1. Fixed incorrect installation guide in the README for example/nbee
2. Deleted base64-encoded image data from test / example notebooks and updated "golden" files:
   2.1. example/nbee/notebook.ipynb had 2 base64 images and weighted a whopping 273 KB. That's a lot of useless data to move around, considering that nbee is just an example for documentation purposes.
   2.2.. testdata/notebook.ipynb had 1 base64 image, which took up ~85% of its size -- that that image is part of the golden-file test is not essential, as rendering of images from a mime-bundle is well-tested in other unit tests
   2.3. Finally, because of so much .ipynb kilobytes, Github would assume nb was a predominantly-Jupyter-notebook-project, which is just not right :)