Extension of javascript abstract plugin class.

Allows to easily integrate custom plugin menu item(s) with admin toolbar.
Menu is automaticaly included if plugin class implements getMenu() abstract method.

Example (pimcore.plugin.poll class):

```javascript
getMenu: function(){
    return new Ext.menu.Item({
        text: t("polls"),
        iconCls: "poll_plugin_menu_icon",
        hideOnClick: false,
        menu: [{
            text: t("create_poll"),
            iconCls: "poll_plugin_menu_new",
            handler: this.openCreateTab
        },{
            text: t("manage_polls"),
            iconCls: "poll_plugin_menu_list",
            handler: this.openListTab
        },{
            text: t("settings"),
            iconCls: "poll_plugin_settings",
            handler: this.openSettingsTab
        }]
    });
}
```
PS: If this code will be accepted i will update plugin anatomy section in docs.
PS2: I think that this aproach could replace existing <pluginIframeSrc/> option for plugin settings management.