Tutorial how to efficiently fix a production issue.

The issue in question is a rounding error in the method de.pdark.tutorial.hotfix.base.Order.collectOrder(User, long)

de.pdark.tutorial.hotfix.base contains the code with the bug, de.pdark.tutorial.hotfix.commit* contains each git "commit".

This makes it more efficient for me to write the tutorial. In the real world, everything would happen in de.pdark.tutorial.hotfix.base.