Major refactorings regarding this module, and how it's used in
nbshell, is needed and planned. For more info on that, the file
'notes' here is an irc session where it was discussed. First part is
done now (the new cell class and accompanying methods in the notebook
class), further things (move of nbshell IPythonLog to nbdoc, making
the block system in nbdoc to replace nbshell Sheet, etc.) are still in
planning.