Allow using elements in logo attribution options

This commit lets the user use an element object instead of a {src: href} object for an attribution logo when creating a map. This opens a lot of possibilities for that logo, for example setting the target to force the logo to open in a new tab when clicked.

examples/custom-icon.js

@@ -0,0 +1,27 @@
+goog.require('ol.Map');
+goog.require('ol.View');
+goog.require('ol.layer.Tile');
+goog.require('ol.source.OSM');
+
+var logoElement = document.createElement('a');
+logoElement.href = 'http://www.osgeo.org/';
+logoElement.target = '_blank';
+
+var logoImage = new Image();
+logoImage.src = 'http://www.osgeo.org/sites/all/themes/osgeo/logo.png';
+
+logoElement.appendChild(logoImage);
+
+var map = new ol.Map({
+  layers: [
+    new ol.layer.Tile({
+      source: new ol.source.OSM()
+    })
+  ],
+  target: 'map',
+  view: new ol.View({
+    center: [0, 0],
+    zoom: 2
+  }),
+  logo: logoElement
+});