Allow for styling of ToolbarAndroid's overflow icon

Summary: Fixes #2858.

Here's a screenshot of the custom overflow icon in action:
<img width="638" alt="overflow" src="https://cloud.githubusercontent.com/assets/1309177/10567090/693e395a-75ae-11e5-84cd-20b19149c620.png">
Closes https://github.com/facebook/react-native/pull/3497

Reviewed By: svcscm

Differential Revision: D2559787

Pulled By: foghina

fb-gh-sync-id: f188711ec094af3fa307722527f22aefff722e64

Libraries/Components/ToolbarAndroid/ToolbarAndroid.android.js

@@ -103,6 +103,10 @@ var ToolbarAndroid = React.createClass({
      * Callback called when the icon is selected.
      */
     onIconClicked: ReactPropTypes.func,
+    /**
+     * Sets the overflow icon.
+     */
+    overflowIcon: optionalImageSource,
     /**
      * Sets the toolbar subtitle.
      */
@@ -135,6 +139,9 @@ var ToolbarAndroid = React.createClass({
     if (this.props.navIcon) {
       nativeProps.navIcon = resolveAssetSource(this.props.navIcon);
     }
+    if (this.props.overflowIcon) {
+      nativeProps.overflowIcon = resolveAssetSource(this.props.overflowIcon);
+    }
     if (this.props.actions) {
       nativeProps.actions = [];
       for (var i = 0; i < this.props.actions.length; i++) {
@@ -169,6 +176,7 @@ var toolbarAttributes = {
   actions: true,
   logo: true,
   navIcon: true,
+  overflowIcon: true,
   subtitle: true,
   subtitleColor: true,
   title: true,