Pybryt

* Changed `pybryt.utils.save_notebook` to use `IPython.display.publish_display_data` instead of
`IPython.display.display`
* Added tracing control with `pybryt.tracing_off` and `pybryt.tracing_on`
* Changed `pybryt.utils.save_notebook` to only force-save in the classic Jupyter Notebook interface
* Added a JSON-friendly output for results with `pybryt.ReferenceResult.to_dict`

* Added Cython and ipykernel to installation dependencies

* Patched bug in `UnassignedVarWrapper` from fix in v0.0.2

* Changed timestamp to step counter in execution trace function
* Fixed `UnassignedVarWrapper` so that variables in boolean operations are only evaluated when
necessary

* Initial release