- `Subscriber::drop_span` in favor of new `Subscriber::try_close` (#168)

Added

- `Into<Option<&Id>>`, `Into<Option<Id>>`, and
  `Into<Option<&'static Metadata<'static>>>` impls for `span::Current` (#170)
- `Subscriber::try_close` method (#153)
- Improved documentation for `dispatcher` (#171)