Adding attribution control and attribution.

src/ol/layer/Layer.js

@@ -4,4 +4,19 @@ goog.provide('ol.layer.Layer');
  * @constructor
  * @export
  */
-ol.layer.Layer = function() {};
+ol.layer.Layer = function() {
+    
+    /**
+     * @type {string}
+     * @protected
+     */
+    this.attribution_;
+    
+};
+
+/**
+ * @return {string}
+ */
+ol.layer.Layer.prototype.getAttribution = function() {
+    return this.attribution_;
+};