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.

README.md

@@ -117,16 +117,21 @@ Specifies the kind of report to be generated. It defaults to text.
 
 ##### text
 ```
-Time(avg)	Requests	Success		Bytes(rx/tx)
-152.341ms	200		    17.00%		251.00/0.00
-
-Count:		49	30	39	48	34
-Status:		500	404	409	503	200
-
+Requests      [total]               1200
+Latencies     [mean, max, 95, 99]   223.340085ms, 7.788103259s, 326.913687ms, 416.537743ms
+Bytes In      [total, mean]         3714690, 3095.57
+Bytes Out     [total, mean]         0, 0.00
+Success       [ratio]               55.42%
+Status Codes  [code:count]          0:535  200:665
 Error Set:
-Server Timeout
-Page Not Found
+Get http://localhost:6060: dial tcp 127.0.0.1:6060: connection refused
+Get http://localhost:6060: read tcp 127.0.0.1:6060: connection reset by peer
+Get http://localhost:6060: dial tcp 127.0.0.1:6060: connection reset by peer
+Get http://localhost:6060: write tcp 127.0.0.1:6060: broken pipe
+Get http://localhost:6060: net/http: transport closed before response was received
+Get http://localhost:6060: http: can't write HTTP request on broken connection
 ```
+
 ##### json
 ```json
 {
@@ -172,14 +177,9 @@ func main() {
   duration := 4 * time.Second
 
   results := vegeta.Attack(targets, rate, duration)
+  metrics := vegeta.NewMetrics(results)
 
-  totalTime := time.Duration(0)
-  for _, result := range results {
-    totalTime += result.Timing
-  }
-  meanTime := time.Duration(float64(totalTime) / float64(len(results)))
-
-  fmt.Printf("Average timing: %s", meanTime)
+  fmt.Printf("Mean latency: %s", metrics.Latencies.Mean)
 }
 ```