Nodebook

This release is a bugfix for function support in Python 3. In v0.2.0, the new `arg` ast node was not parsed correctly, leading to functions not being usable in nodebook.

This release adds support for Python 3. Special thanks to jni for doing the conversion!

This release also brings Nodebook more in line with standard patterns for installing Jupyter extensions, which results in two important changes:
* It is now required to manually register the extension with Jupyter after installing Nodebook using `jupyter nbextension install --py nodebook`
* The extension import command has changed from `%load_ext nodebookext` to `%load_ext nodebook.ipython`