loadUrl enhancements (#134)

`loadUrl` enhancements

- Make `options` parameter optional
- Allow partial overriding of instance options when calling `loadUrl` directly
- Make `requestOptions` optional
- Document `loadUrl` usage and provide examples

lib/util/clone.js

@@ -0,0 +1,12 @@
+module.exports = function(obj) {
+  if (null === obj || "object" !== typeof obj) {
+    return obj
+  }
+  var copy = obj.constructor()
+  for (var attr in obj) {
+    if (obj.hasOwnProperty(attr)) {
+      copy[attr] = obj[attr]
+    }
+  }
+  return copy
+}