* fixed instance register without hostname and pid

* format code using gofmt -s

* Documenting go2sky

 * Update README.md
 * Add pakcage document

* Update README.md

* Update comments of NewGRPCReporter

* Adding more unit test cases including

 * noop test
 * gRPC reporter test
 * tracer parameter test
 * and so on

* Add http plugin

* Add essentail tags

* Add component

* Add log reporter for examples

* Polish codes

* noop test
 * gRPC reporter test
 * tracer parameter test
 * and so on

* Add downstream propagation extraction

* Add upstream propagation

* Add span context that links all spans in a local trace context

* Implement Span

- Add gRPC lib
- Add skywalking proto and generate *.pb.go files
- Implement service and instance register