Add analysis support for Hansen gain, cover, loss

OK! We have the Hansen layers for analyzing gain, cover, and loss and it's deployed to wip. :D

Here's the result of [loss by year](http://wip.gfw-apis.appspot.com/datasets/hansen?layer=loss&geom={%22type%22:%22Polygon%22,%22coordinates%22:[[[102.65350,-0.73499],[103.75488,-0.89153],[104.14764,-1.57527],[102.77161,-1.47368]]]}&bust=1) for supplied GeoJSON polygon:

```json
{
   "geom":{
      "type":"Polygon",
      "coordinates":[
         [
            [
               102.65349999999999,
               -0.73499000000000003
            ],
            [
               103.75488,
               -0.89153000000000004
            ],
            [
               104.14764,
               -1.5752699999999999
            ],
            [
               102.77161,
               -1.4736800000000001
            ]
         ]
      ]
   },
   "percent":{
      "2002":1.9966222915041087,
      "2003":1.1870389367657674,
      "2001":1.6547742563060646,
      "2006":4.0649652906080593,
      "2007":4.5303926107208508,
      "2004":4.6090374476645835,
      "2005":2.7850242630506181,
      "2008":6.2511960612089847,
      "2009":4.7399017399539272,
      "2011":0.80362753029235201,
      "2010":0.78036573932823039,
      "2012":3.9541263957490189
   },
   "area":{
      "2002":208.33516856552924,
      "2003":123.8601602502595,
      "2001":172.6654435805728,
      "2006":424.15394871407898,
      "2007":472.71840660045905,
      "2004":480.9245081906646,
      "2005":290.60003074724784,
      "2008":652.27358759340586,
      "2009":494.57938648662491,
      "2011":83.85355492612284,
      "2010":81.426331128075503,
      "2012":412.58859659799919
   }
}
```

Here's the result of summing over the study years 2000-2012 for [gain, cover, and loss](http://wip.gfw-apis.appspot.com/datasets/hansen?layer=sum&geom={%22type%22:%22Polygon%22,%22coordinates%22:[[[102.65350,-0.73499],[103.75488,-0.89153],[104.14764,-1.57527],[102.77161,-1.47368]]]}&bust=1) for supplied GeoJSON polygon:

```json
{
   "geom":{
      "type":"Polygon",
      "coordinates":[
         [
            [
               102.65349999999999,
               -0.73499000000000003
            ],
            [
               103.75488,
               -0.89153000000000004
            ],
            [
               104.14764,
               -1.5752699999999999
            ],
            [
               102.77161,
               -1.4736800000000001
            ]
         ]
      ]
   },
   "percent":{
      "loss_sum":37.436577268474423,
      "treecover_2000":86.387937243368555,
      "gain":21.351877283895057
   },
   "area":{
      "loss_sum":3906.2749469086475,
      "treecover_2000":9014.04614392248,
      "gain":2227.9361359730915
   }
}
```