Plaited

plaited is now ready to live on PyPI, though it may change quite a bit before 1.0.0.

- fixed bug that tried to execute code before waiting for Jupyter kernel, causing timeouts
- Python kernel now comes bundled with plaited (ipykernel)
- added test that actually executes a code block in a document, and checks its output

plaited now works well enough that I'd like to start publishing it on TestPyPI. I'll add install instructions to README.md once it's successfully published.