Add abstract classes for tracing following the [java client](https://github.com/open-telemetry/opentelemetry-java/tree/master/api/src/main/java/io/opentelemetry/trace) and the (WIP) [spec](https://github.com/open-telemetry/opentelemetry-specification). The API should also include no-op implementations for some classes, see the java client for details.
Add abstract classes for tracing following the java client and the (WIP) spec.
The API should also include no-op implementations for some classes, see the java client for details.