Ignore iPython left-overs

A `Untitled0.ipynb` file was left in the [cqlengine-0.18.1.tar.gz](https://pypi.python.org/packages/source/c/cqlengine/cqlengine-0.18.1.tar.gz#md5=a9cb831315c8a5f2e370a7d32fc77cb5) archive on PyPi.