Allow the user to retrieve the last ``rows_per_page`` items regardless of the time

This should fix #23
Reorganize the app to make the datetime stuff testable.
Some tests for the datetime stuff.
Change the docs for one of the datetime combinations.
Fix up our logic to get all tests passing.
PEP8
Merge pull request #42 from fedora-infra/retrieve_last_items

Allow the user to retrieve the last ``rows_per_page`` items regardless o...