Only set the package version in __init__.py

At the moment we're holding the package version in two places: `setup.py` and `__init__.py`. We need to keep the version number in the init file for a number of reasons, but we could add some magic to avoid hard-coding it again in the setup file. Either that or soon or later I (or someone else) will forget to bump both numbers before a release. And also, DRY.