the public API now uses i32/u32 instead of int/uint for integers.

closes #1.
major API surgeries.

- added a new example.
- reexported all public APIs in the crate root.
- made all constructors fail on the invalid arguments by default;
  `*_opt()` variants have been added for the original behavior.
- same for `DateZ::{succ,pred}`.
- fixed a missing overflow check from `TimeZ::from_hms_{milli,micro}`.