add an OSM layer constructor to the API

test/spec/api/layer/osm.test.js

@@ -0,0 +1,12 @@
+describe('ol.layer.osm', function() {
+
+    describe('create an OSM layer', function() {
+
+        it('returns an ol.layer.OSM instance', function() {
+            var layer = ol.layer.osm();
+            expect(layer).toBeA(ol.layer.OSM);
+        });
+
+    });
+
+});