Add Tracer API
Add Tracer API

Closes #28
open-telemetry/opentelemetry-js,open-telemetry/opentelemetry-js-contrib,open-telemetry/opentelemetry-js-contrib,open-telemetry/opentelemetry-js-contrib,open-telemetry/opentelemetry-js,open-telemetry/opentelemetry-js,open-telemetry/opentelemetry-js,open-telemetry/opentelemetry-js-contrib