language changes: MaybeOwned is deprecated, long live CowString!
better documentation, some additional APIs.

- `Date/Time - Duration` is now supported. (duh!)
- `with_offset` methods have been added.
- `LocalResult` now implements common traits.
- `LocalResult` has several methods to propagate errors.
  this makes the initialization from untrusted sources easier
  (`off.ymd_opt(y,m,d).and_hms_opt(h,n,s).single()`).
0.1.3: better offset conversion and additional APIs.

- better documentation :)
- `Offset::name` now returns `SendStr`.
- `Date/Time - Duration` is now supported.
- `with_offset` methods have been added.
- `LocalResult` now implements common traits.
- `LocalResult` has several methods to propagate errors.
  this makes the initialization from untrusted sources easier.