Release v0.5 to PyPI

Thanks for this great project! However, when I did `pip install ipynb` I got version 0.4, which does not correctly comment out markdown cells when converting into code (fixed in commit b266e352bf1726477129270d4530ed2df7bdc580, which lies between 0.4 and 0.5). Currently one must do `pip install -e git+https://github.com/ipython/ipynb#egg=ipynb` to get the fixed version.

NB, the PyPI page also has an old project URL http://github.com/yuvipanda/ipynb instead of http://github.com/ipython/ipynb .