SpanBuilder interface for #5

[cwe1ss]

As an example for #5, this would introduce a ISpanBuilder interface and remove the existing StartSpanOptions and the existing SpanBuilder implementation.

This is exactly how the Java version is implemented.

I favor having the interface over an actual implementation to let trace implementations optimize the behavior without having users to update their OpenTracing package.

[cwe1ss]

I will merge this on Tuesday If there are no complaints by then.

[dawallin]

As I commented in #5 I prefer the StartSpanOptions, and having the builder as an optional extension for a fluent API style. But if you think the SpanBuilder is better I won't object.

If we go for the SpanBuilder then this PR looks perfectly fine.

[cwe1ss]

Ok, I'll merge it!

Btw, it would be possible to go the other way and offer non-breaking extension methods like this one in the future:

public static ISpan StartSpan(this ITracer tracer, string operationName, SpanReference reference)
{
    return tracer.BuildSpan(operationName).AddReference(reference).Start();
}

// Usage
tracer.StartSpan("get_account", SpanReference.ChildOf(parent));

// But it would even be more characters than the current solution
// so there's probably not much point... :-)
tracer.BuildSpan("get_account).AsChildOf(parent).Start();