fix next_point for single-repetition recurrence
fix prev_point for single repetition recurrences
add more recurrence tests
Merge pull request #28 from benfitzpatrick/fix-next-recurrence-null-interval

Fix get next point for single-repetition recurrences
Merge pull request #29 from benfitzpatrick/strftime_strptime

Implement subset of strftime/strptime POSIX standard
Merge branch 'master' of github.com:metomi/isodatetime into strftime_strptime