Tracing call hierarchies

Child request should be filled up by additional tracing info to be properly tracked. It also should be properly sampled if it leaves origin actor system (it can be done in HTTP layer using Spray integration, or in any other layer using corresponding API call). Depending on type of child request, you can use following approaches to correctly prepare it.

Any class mixed with TracingSupport

It's a simple case that requires you to mark child requests using trace.createChild(parentRequest, childRequest). This case is covered in following example.

HttpRequests made using Spray client

WIP, see #28