ScribeClient: log pushes to the "queue"

https://wikia-inc.atlassian.net/browse/PLATFORM-9

@Wikia/platform-team / @wladekb 

Adding logging of Scribe pushes, first step towards documenting the data flow in our system :)

```json
{
  "@timestamp": "2014-09-24T15:49:19.360910+00:00",
  "@message": "Scribe",
  "@fields": {
    "db_name": "plpoznan",
    "city_id": "5915",
    "url": "/wiki/Gzik?action=purge",
    "ip": "10.14.30.102",
    "http_method": "POST",
    "server": "poznan.macbre.wikia-dev.com",
    "request_id": "mw5422e7fc7c60a8.41102147"
  },
  "@context": {
    "cmd": "send",
    "category": "varnish_purges",
    "caller": "ScribePurge:onRestInPeace"
  }
}
```