Update setup.py to pull version number from __init__.py

When importing libraries in ipython/python, I get an error:

```
>>> from democritus_dates import *
Unable to find a version number for "democritus_dates". This likely means the library was not installed properly. Please re-install it and, if the problem persists, raise an issue here: https://github.com/democritus-project/democritus-dates/issues.
```

Start by comparing setup.py with democritus_dates/__init__.py to see how the package version is currently defined and discovered. Verify the package installation and import path in IPython/Python; done means the installed library reports its version without the shown error.

Written by the indexing model from the issue text.