* 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)
* trace sdk initial commit.
* fix imports and comments.
* remove tracestate
* split trace.go
* add attribute over limit test.
* add comments and restructure span.go and tracer.go
* refactor MessageEvent
* defer unlock
* some more cleanup in span.go
* rename *MessageEvent* to *Event*
* cleanup comments in trace_test.go
* fix typos.
* return full string ID for traceID and spanID.
