Need a way to access active span of automatic instrumentation

[pglombardo]

If a user wants to sprinkle in OpenTracing among existing automatic instrumentation, they need a way to access the currently active span to inherit context from.

[pglombardo]

This was added in #43. Current context can be retrieved with:

import instana
context = instana.internal_tracer.current_context
span = instana.internal_tracer.start_span("child_span", child_of=context)
...