WIP: propagation scopes

[tpolecat]

This updates put to take a Propagation value that explains how the fields should be propagated to child spans. Resolves #17

Value	Meaning
Propagation.None	fields will not be propagated to child spans
Propagation.Local	fields will be propagated to child spans, but will not cross network boundaries
Propagation.Extended	fields will be propagated to child spans, and across network boundaries

Partially implemented for Honeycomb. Parameter currently ignored for other back ends.