* Allow specifying custom timestamps for events
Adding event with timestamp is not yet a part of the OpenTelemetry
specification, but this function will come in handy when implementing
the OpenTracing bridge.
* Add opentracing bridge, wrapper tracer and migration interfaces
There are some features missing - setting up links and span kind;
context propagation will only work between two OpenTracing bridges.
* Add some tests for the opentracing bridge
The tests mostly check various aspects of the cooperation between
OpenTracing and OpenTelemetry APIs.
