Timestamp formatting

---

examples/time/formatting.cr

time = Time.utc(2016, 2, 15, 10, 20, 30)
puts time.to_s("%Y-%m-%d %H:%M:%S %:z")
puts time.to_s("%Y-%m-%d")

2016-02-15 10:20:30 +00:00
2016-02-15