Add ReportDateType "enum" class.

ReportDateType provides three global instances: +daily, +weekly and
+yearly and will be a place to gather data and logic related to
daily, weekly and yearly reports.  Currently, ReportDateType only
contains -name (with values of @"Daily", @"Weekly" and @"Yearly"
respectively).

Added a reportDateType property to ReportCategory and added a
corresponding field to its -init method, removing the corresponding
NSString dateType parameter.  Changed the -isDaily, -isWeekly and
-isYearly methods to compare against ReportDateType rather than do
a string comparison against dateType.