Add more granular HTTP status code metrics
* ZuulStatusCodeClass tracks 1xx, 2xx, 3xx, 4xx, 5xx
* ZuulStatusCode tracks "interesting" codes (200, 201, 202, 301, 302, 304, 400, 401, 403, 404, 410, 500, 503) and dumps the rest in 1xx/2xx/3xx/4xx/5xx buckets
Merge pull request #84 from mattrjacobs/specific-http-status-reporting

Add more granular HTTP status code metrics