Add delegate forging statistics

[ghost]

In order to assist with delegate forging reward distribution, I think it would be helpful to extend the delegates forging API endpoint with forging statistics.

GET /api/delegates/forging/delegateStatistics?username=username&day=05&month=05&year=2016
GET /api/delegates/forging/delegateStatistics?username=username&month=05&year=2016
GET /api/delegates/forging/delegateStatistics?username=username&year=2016

{
  "success": true,
  "sum": "Forged amount. Integer"
}

[dakk]

I think it's better to extend getForgedByAccount with two optional parameters "start" and "end" that indicate the timeslice (in unixtime) of the query