Extend Metrics with max and 95th, 99th percentiles

Implements a new format for the text reporter with more information
than before.
Also Rename Result.Timing to Result.Latency and related names.
Adapt README.md examples to account new text reporter format and
library changes.

lib/attack.go

@@ -56,7 +56,7 @@ func hit(req *http.Request, res chan Result) {
 	r, err := client.Do(req)
 	result := Result{
 		Timestamp: began,
-		Timing:    time.Since(began),
+		Latency:   time.Since(began),
 		BytesOut:  uint64(req.ContentLength),
 	}
 	if err != nil {