fix next_point for single-repetition recurrence
fix prev_point for single repetition recurrences
add more recurrence tests
implement subset of strftime/strptime POSIX standard
separate out timezone functions, custom parsing
Merge pull request #28 from benfitzpatrick/fix-next-recurrence-null-interval

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

Implement subset of strftime/strptime POSIX standard