This change fixes a compiler warning and changes the way a PPRScheduleTime computes its description string.

PPRScheduleTime.m - use a DateFormatter etc., rather than a crude stringWithFormat;
PPRTestIIntialiser.m [sic] - fix the warning;
PPRScheduleTimeTests.m - added, for a crude test.
PPRScheduleTime
- change the description routine to use the calendar in the PPRScheduler.

PPRScheduleTimeTests
- refactor existing set-up code so it can be shared by test routines.
- introduce a test to assert different calendars by setting the calendar property in the singleton, and then "duplicate" the calculation and check it matches.
PPRScheduleTimeTests:
- made to assert the Gregorian calendar rather than relying on it to be the current calendar;
- finish the test that must duplicate the time calculation using another calendar.
PPRScheduleTimeTests:
- Fix testDuplicatingCalculation
Merge pull request #11 from dvincent/fix_warning_and_nonlocalisable_description

Fix a build warning & change how a PPRScheduleTime describes itself