* Add golint to linters and resolve issues.
I decided to remove constructors for some of the propagation types
because the constructors can be reduced to either using the zero value
or a single, non optional member.
* Enable gofmt and commit fixes
* rename finish -> end
* missed a few finish -> end spots
* change end back to finish for the bridge span in the openTracing bridge
* fixed grammar, ran make
* 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.
