Improvements to the 'open-source-ability' of this library

I have made various small improvements to this library to make it more usable by third parties, partly due to using it myself in a project of my own. These include:

* setup.py now uses proper PEP-compliant version numbers (this will change how LaterPay handles its dependency on django-flows to behave just like a regular third party library, rather than considering it in the same way as an in-house library)

* Documentation (mainly README.md) has improved, although still requires work

* There were some bugfixes and improvements in the database-based task store, such as implementing task timeout properly and adding cleanup tools to delete old state.