Bump version to 0.4.0. (#358)

Bump version to 0.3.0. (#274)

Don't depend on gRPC private header. (#174)

GetSpanFromServerContext() comes from <grpcpp/opencensus.h> as of
grpc/grpc#15984

Make exporters/trace/stackdriver more consistent with the stats one. (#…

…105)

- This simplifies the public API.
- Remove stackdriver_exporter_test for now, it wasn't testing anything.
- Add README pointing at the stats/ README.

Make trace/ Exporter API consistent with the stats/ one. (#9)

Also:
- Add sampler_test.
- Replace Span's default constructor with BlankSpan().
- Add BlankSpan test.
- Add FullSpanTest that verifies SpanData.
- Initialize threads after the rest of the exporter, to avoid racing.