ERP5 kernel basic info/workflow:

1. User enters code on notebook cell and executes
2. Code is sent to kernel via websockets
3. Kernel sends request to ERP5
4. Code is executed by ERP5 and the result is returned back via request.
5. Result is received and rendered on the notebook frontend.
6. Other message formats such as error and status are also conveyed by the Kernel.

[ kirr: in IPython notebook speak kernel is something that allows IPython
  notebook server side to talk to execution backend. ERP5 kernel is a thing that
  allows ipython notbook to talk to ERP5 (with help on-ERP5-server special bt5
  installed which accepts and executes commands).

  The bt5 to handle notebook calls on ERP5 side - erp5-data-notebook - is
  proposed to be merged into erp5.git on nexedi/erp5!29 ]

/initially-reviewed-by @kirr, @Tyagov  (in a lot of places, last time on nexedi/slapos!33)