Issue 9: Adding spray-metrics

- Metrics are now instantiated using a factory
- The CounterMetric is the "interesting" one to look at.
  - It defaults to counting successes and failures
  - If you call 'countingRejections' on it, it will construct a new factory that
    will also count rejections
- There's a decent amount of code duplication at this point but I want to get
  some eyes on it before moving much further
- It's not hyper-flexible at the moment