ERP5 Jupyter kernel: capture print without messing with sys.stdout

Instead of swapping sys.stdout temporally, which is not a nice thing to do in an ERP5 cluster architecture where code can run in different nodes, an AST processor was created to modify print calls and redirect them to a custom class that will collect all the printed data.

The idea of this implementation is based on Script (Python) objects code.

@Tyagov, please review when you can.

Depends on !144 (merged).

This closes the following bug report: https://nexedi.erp5.net/bug_module/20160718-2079490