* 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

* Creating noop span when tracer is not inited
 * If the current context contans noop span, other spans are all noop spans

* 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