fixes `menu` type, add icon support

### `menu` type support
As described by the documentation, and as generated by the scaffolding- the name of the property should be `menu` instead of `path`.
This PR is allow you to set `menu` or `path`.

```js
  Investor.menus.add({
    title: 'investors manage',
    link: 'investors.admin',
    icon: 'mean-admin/assets/img/icons/settings.png',
    roles: ['admin'],
    menu: 'admin'
  });
  ```

### `icon` support
Also, this PR enable using the `icon` property(as used in the `mean-admin`)