Output placeholders are incompatible with nbformat spec

[3coins]

The placeholder output values for stream and error are incompatible with nbformat, which causes error in loading these files using tools like nbconvert.

Expected format for stream

{
    "output_type": "stream",
    "name": "stdout",  # or stderr
    "text": "[multiline stream text]",
}

Expected format for error

{
    'output_type': 'error',
    'ename' : str,   # Exception name, as a string
    'evalue' : str,  # Exception value, as a string

    # The traceback will contain a list of frames,
    # represented each as a string.
    'traceback' : list,
}

[3coins]

Fixed by #136