Provide safe defaults for span creation

[c24t]

From #11 (review).

The tracing API should describe the default options for span creation with missing arguments. We can't do this unilaterally, see open-telemetry/opentelemetry-specification#141 to add this behavior to the spec.

[c24t]

This was fixed in later changes to the tracer, we don't throw at all in the API now.