Improve and correct how dates for downloaded reports are logged.

Changed date calculation for monthly reports to force date to the first of
the month.  Changed date formatting for log messages to show day-month-year
for daily and weekly reports, month-year for monthly reports and only year
for yearly reports.  Added report subtype to log message.

In NSCalendar (Autoindigestion), changed -twelveMonthsAgoForDate: to
-twelveFirstOfTheMonthsAgoForDate: and added method
-previousFirstOfTheMonthForDate:.

In ReportCategory, added -reportDescriptionWithDate: method that
extracts the report description logic, including date formatting, out of
AutoindigestionJob.

Added -formattedDateForDate: method to ReportDateType that handles the
correct date formatting for each report date type.

TODO: ReportCategory -isValidReportDate: method should be moved to
ReportDateType.