Init SkywalkingTracer with transport configuration

[Filippo Balicchia]

Hi. I don't know if this is the right place.. if no sorry in advance.

I've instrumented a tiny app with OpenTracing api and I'd like to swith
system in favor of Skywalking.
In SkywalkingTracer I can't find a way to inizialized it with a sort of
sender that send data to Skywalking
collector.

```
Tracer tracer = new SkywalkingTracer();
        GlobalTracer.register(tracer);

        ActiveSpan parent =
GlobalTracer.get().buildSpan("Ciao").startActive();
        for (int i = 0; i < 5; i++)
        {
            parent.log("Start child span " + i);

            ActiveSpan child = GlobalTracer.get()
                .buildSpan("Mondo")
                .asChildOf(parent).startActive();
            try
            {
                Thread.sleep(3000);
                System.out.println("wakeup.... ");
                child.close();
            }
            catch (InterruptedException e)
            {
                //NOOP
            }
        }
        parent.close();
```
Is there an API that I missed it ?
Thanks for attention

--Filippo