Document fab icon tinting

docs/adding-buttons-to-the-navigator.md

 Each screen can contain a single Fab which is displayed at the bottom right corner of the screen.
 
 * Simple Fab:
+
 ```js
   static navigatorButtons = {
     fab: {
       collapsedId: 'share',
       collapsedIcon: require('../../img/ic_share.png'),
+      collapsedIconColor: 'red', // optional
       backgroundColor: '#607D8B'
     }
   };
 
 * Fab with expanded state
 [Example](https://material-design.storage.googleapis.com/publish/material_v_9/0B8v7jImPsDi-ZmQ0UnFPZmtiSU0/components-buttons-fab-transition_speeddial_02.mp4)
+
 ```js
     fab: {
       collapsedId: 'share',
       collapsedIcon: require('../../img/ic_share.png'),
+      collapsedIconColor: 'green', // optional
       expendedId: 'clear',
       expendedIcon: require('../../img/ic_clear.png'),
+      expendedIconColor: 'red', // optional
       backgroundColor: '#3F51B5',
       actions: [
         {
           id: 'mail',
           icon: require('../../img/ic_mail.png'),
+          iconColor: 'blue', // optional
           backgroundColor: '#03A9F4'
         },
         {