Store the feature's commonly used id

To not clobber the feature's attributes, this is a separate
member property.

test/spec/ol/feature.test.js

@@ -16,6 +16,12 @@ describe('ol.Feature', function() {
       expect(feature.get('foo')).to.be('bar');
     });
 
+    it('can store the feature\'s commonly used id', function() {
+      var feature = new ol.Feature();
+      feature.setFeatureId('foo');
+      expect(feature.getFeatureId()).to.be('foo');
+    });
+
     it('will set the default geometry', function() {
       var feature = new ol.Feature({
         loc: new ol.geom.Point([10, 20]),

test/spec/ol/parser/geojson.test.js

@@ -151,6 +151,7 @@ describe('ol.parser.GeoJSON', function() {
         var first = result[0];
         expect(first).to.be.a(ol.Feature);
         expect(first.get('name')).to.be('Afghanistan');
+        expect(first.getFeatureId()).to.be('AFG');
         var firstGeom = first.getGeometry();
         expect(firstGeom).to.be.a(ol.geom.Polygon);
         expect(ol.extent.equals(firstGeom.getBounds(),
@@ -160,6 +161,7 @@ describe('ol.parser.GeoJSON', function() {
         var last = result[178];
         expect(last).to.be.a(ol.Feature);
         expect(last.get('name')).to.be('Zimbabwe');
+        expect(last.getFeatureId()).to.be('ZWE');
         var lastGeom = last.getGeometry();
         expect(lastGeom).to.be.a(ol.geom.Polygon);
         expect(ol.extent.equals(lastGeom.getBounds(),