Add a metric with maximum and median number of requests from a single domain

* maxRequestsPerDomain
* medianRequestsPerDomain

```
./phantomas.js --url=http://www.cnn.com --modules=domains  --verbose

Requests per domain:
 i.cdn.turner.com: 51 request(s)
 i2.cdn.turner.com: 49 request(s)
 ads.cnn.com: 15 request(s)
 (...)
```