Convert to / from IPython Notebook

As discussed in issue #11, this implements conversion to and from IPython Notebooks.

The IPython codebase contains the required methods for writing strings to notebook format (which is a JSON), and this just uses that.

There is a test reference included (`tests/simple_REF.ipynb`).

Command line usage:

    # list available conversion formats
    Pweave-convert --list-formats
    # convert to ipynb
    Pweave-convert -i noweb -f notebook file.mdw