* Merge two event methods in span API
There was an agreement to get rid of the Event interface and
consolidate the two methods for adding events into one. See #57.
* Eliminate the use of the Event interface
There is no need for the SDK to provide the implementation of the
Event interface - it is used nowhere.
* Drop the Event interface
It's dead code now.
* Make it possible to override a finish timestamp through options
Opentracing to opentelemetry bridge will certainly use this feature.
* Obey the start time option
* Add tests for events and custom start/end times
* 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.
