Add collector which convert JSON into flat metrics

Simple collector to get JSON by url and convert it into flat metrics. Could be used in common cases.


For example, JSON below
```json
{
  "status": {
    "main": {
      "3xx": 0,
      "4xx": 7,
      "5xx": 0,
      "xxx": 0,
      "1xx": 0,
      "2xx": 0
    },
    "_total": {
      "3xx": 0,
      "4xx": 0,
      "5xx": 7,
      "xxx": 0,
      "1xx": 0,
      "2xx": 0
    }
  },
  "timings": {
    "main": {
      "1000-inf": 0,
      "0-100": 7,
      "undef": 0,
      "100-500": 0,
      "500-1000": 0
    },
    "_total": {
      "1000-inf": 0,
      "0-100": 7,
      "undef": 0,
      "100-500": 0,
      "500-1000": 0
    }
  }
}
```

will convert into graphite metrics like

```
.status.main.3xx=0
.status.main.4xx=7
...
.status._total.5xx=0
...
.timings._total.1000-inf=0
...
and etc
```