! core: replace Option[TraceContext] by empty object pattern and implement basic segments with renaming.
= all: upgrade to be compatible with the latest code in core
+ spray: external naming for traces and segments, related to #65
+ play: external naming for traces and segments, related to #65
! spray: rename the default implementation of SprayNameGenerator
Merge branch 'wip/improve-trace-context-and-segments-api', closes #65