* 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)
* Refactor SDK Sampler API to conform to Spec
* Sampler is now an interface rather than a function type
* SamplingParameters include the span Kind, Attributes, and Links
* SamplingResult includes a SamplingDecision with three possible values, as well as Attributes
* Add attributes retruned from a Sampler to the span
* Add SpanKind, Attributes, and Links to API Sampler.ShouldSample() parameters
* Drop "Get" from sdk Sampler.GetDescription to match api Sampler
* Make spanID parameter in API Sampler interface a core.SpanID
* Fix types and printf format per PR feedback from krnowak
* Ensure unit test error messages reflect new reality
Co-authored-by: Joshua MacDonald <jmacd@users.noreply.github.com>