Add option to print min/mean/stddev/max response times

I added a command-line option to print concise statistics at the end of the benchmark. Executing photon with the newly-added '-minmax' option produces the following additional output:

```
test responseTime(90%): 1ms
test      min    mean      sd     max
test        0    2.86   12.55      85
```

I'm making this a pull request in case you'd like to merge it into the main line of development.

(I'm not sure 'minmax' is the best option name; feel free to rename)