Request decorators, chains, and repeats

[pcantrell]

This PR covers three related areas of functionality:

• Configuration.decorateRequests(...) replaces beforeStartingRequest, and allows you to replace requests based on configuration instead of just listening to them.
• Request.chained(...) allows you to chain multiple requests together and have them behave as if they were one request.
• Request.repeated() allows you to send a request again, picking up configuration changes.

Putting it all together, this allows you to configure sophisticated service-wide retry behavior. For example, this hypothetical code automatically renews auth tokens and retries unauthorized requests for a whole service:

service.configure {
  $0.config.decorateRequests {
    refreshTokenOnAuthFailure($1)
  }
}

// Refactor away this pyramid of doom however you see fit
func refreshTokenOnAuthFailure(request: Request) -> Request {
  request.chained {
      guard case .Failure(let error) = $0.response where error.httpStatusCode == 401 else {
        return .UseThisResponse
      }

      return .PassTo(refreshToken().chained {
        if case .Failure = $0.response {
          return .UseThisResponse
        } else {
          return .PassTo(request.repeated())
        }
      })
    }
  }
}

func refreshToken() -> Request {
  // request new token, add onSuccess handler to update service config
}

@vdka and @jyounus: This should give you at least some of what you are looking for with #48.

This work is a first draft. Kick the tires!

[jyounus]

Hey @pcantrell, thanks for implementing this functionality. The project I was working on is done now, we ended up using a different API lib to accomplish what we were trying to do. I'm definitely going to use Siesta in the future to give this a try!