Add process tracer and specs

Zipkin::ProcessTracer class with methods ```#configure``` alias to ```#initialize``` in ```RackHandler``` and ```#start_new_trace``` alias to ```#call``` in RackHandler, should be called with a block of work.

```#trace_child``` and ```#trace_internal``` provide tracing of a block of work for as new spans inheriting from a parent span or not, respectively. Both use current process trace id.

```
ZipkinTracer::ProcessTracer
  #configure
    initializes a trace
    requires a service name
    requires a scribe server
  #configure_sample_rate
    when declared
      when greater than 1.0 or less than 0.0
        raises an error
        raises an error
      when between 0.0 and 1.0
        returns the declared value
    when undeclared
      uses a default rate
  trace processes
    #start_new_trace
      instantiates a new TRACEID
      instantiates a new SPANID
      pushes the trace id to the trace
      yields to the block and stops tracing once the block is finished
    #trace_internal_process
      uses the current TRACEID as TRACEID
      does not use a PARENTID
    #trace_child
      uses the current TRACEID as TRACEID
      updates PARENTID using the current SPANID
      creates a new SPANID
      pushes the trace id to the trace
      yields to the block and stops tracing once the block is finished
```