Support filtering based on numeric values

[alexmuller]

Given a request like this:

/data/data-group/data-type?flatten=true&collect=costs%3Asum&group_by=dept&start_at=2014-04-01T00%3A00%3A00Z&period=quarter&end_at=2014-07-01T00%3A00%3A00Z

Which returns something like this:

{
  "data": [
    {
      "_count": 1.0,
      "_end_at": "2014-07-01T00:00:00+00:00",
      "_start_at": "2014-04-01T00:00:00+00:00",
      "costs:sum": 765432,
      "dept": "bat"
    },
    {
      "_count": 1.0,
      "_end_at": "2014-07-01T00:00:00+00:00",
      "_start_at": "2014-04-01T00:00:00+00:00",
      "costs:sum": 0.0,
      "dept": "baz"
    },
    {
      "_count": 1.0,
      "_end_at": "2014-07-01T00:00:00+00:00",
      "_start_at": "2014-04-01T00:00:00+00:00",
      "costs:sum": 123456,
      "dept": "bar"
    },
    {
      "_count": 1.0,
      "_end_at": "2014-07-01T00:00:00+00:00",
      "_start_at": "2014-04-01T00:00:00+00:00",
      "costs:sum": 7896,
      "dept": "foo"
    }
  ]
}

It would be useful to apply a filter parameter of costs:sum > 0 to exclude the 0 value from the response.