Support elements in icon attribute in IconButton

This makes IconButton more flexible. It also makes icon elements work
when used with leftIcon/rightIcon attributes in AppBar.

components/button/IconButton.js

@@ -65,7 +65,7 @@ const factory = (ripple, FontIcon) => {
       };
 
       return React.createElement(element, props,
-        icon ? <FontIcon className={theme.icon} value={icon}/> : null,
+        icon ? typeof icon === 'string' ? <FontIcon className={theme.icon} value={icon} /> : icon : null,
         children
       );
     }

components/button/readme.md

@@ -23,6 +23,7 @@ const TestButtons = () => (
     <Button icon='add' floating />
     <Button icon='add' floating accent mini />
     <IconButton icon='favorite' accent />
+    <IconButton icon={<GithubIcon />} accent />
     <IconButton primary><GithubIcon /></IconButton>
     <Button icon='add' label='Add this' flat primary />
     <Button icon='add' label='Add this' flat disabled />

docs/app/components/layout/main/modules/examples/button_example_1.txt

@@ -15,6 +15,7 @@ const ButtonsTest = () => (
     <Button icon='add' floating />
     <Button icon='add' floating accent mini />
     <IconButton icon='favorite' accent />
+    <IconButton icon={<GithubIcon />} accent />
     <IconButton primary><GithubIcon /></IconButton>
     <Button icon='add' label='Add this' flat primary />
     <Button icon='add' label='Add this' flat disabled />