Duration on Metrics and depenendent Reporters

This is real duration, calculated by subtracting the last Result's
timestamp with the first Result's timestamp. Garbage
collection and go routine scheduling delays can affect this number.
Fixes #29

README.md

@@ -118,6 +118,7 @@ Specifies the kind of report to be generated. It defaults to text.
 ##### text
 ```
 Requests      [total]               1200
+Duration      [total]               1.998307684s
 Latencies     [mean, 95, 99, max]   223.340085ms, 326.913687ms, 416.537743ms, 7.788103259s
 Bytes In      [total, mean]         3714690, 3095.57
 Bytes Out     [total, mean]         0, 0.00
@@ -149,6 +150,7 @@ Get http://localhost:6060: http: can't write HTTP request on broken connection
     "total": 0,
     "mean": 0
   },
+  "duration": 1998307684,
   "requests": 1200,
   "success": 0.11666666666666667,
   "status_codes": {