* Update License header for all source files
- Add Apache 2.0 header to source files that did not have one.
- Update all existing headers dated to 2019 to be 2020
- Remove comma from License header to comply with the Apache 2.0
guidelines.
* Update Copyright notice
Use the standard Copyright notices outlined by the
[CNCF](https://github.com/cncf/foundation/blob/master/copyright-notices.md#copyright-notices)
* Provide conformance tests for tracers
The test harness may be used to ensure that a given tracer behaves
according to the expectations set by the API.
* Add `ToMatchError` matcher
* Use DeepEqual to compare unknown types in matchers
Unlike basic `==`/`!=`, `reflect.DeepEqual` can compare arbitrary
types (e.g., `[]string` to `[]string`)
* Use struct instead of string for test context key
