Example absences should be randomly distributed through the year

Currently, the auto-generated set of example absences uses fixed values for the date/time (eg '2014-01-02 14:00:00', '2014-01-5'). When viewed on the calendar, this means that a handful of dates appear as black (because they have multiple absence records for the same person – which is an obscure corner-case).

We need the example data to include the typical case in which absences are spread throughout the year. (Aside: it is nice to also include some corner-cases, but they should be a small part of the dataset – not the entire dataset.)