Only capture print calls when directed at standard output

[almarklein]

In order to prevent that calls to print() are lost, jupyter_rfb overloads builtins.print via a context manager, and places printed messages below the widget.

The problem, as indicated by @QuLogic in a comment in #37, is that it redirects all prints, regardless of the file argument.

Some options:

• Check whether the problem that messages are not shown anywhere still persist in recent versions of Jupyter.
• Check the file argument.
• I don't recall all the details, but is there a sys.stdout that we can capture instead?