Changed ons-toolbar to accept center class element only.
Addded PageView class.
Added 'pre-close', 'pre-open', 'post-close', 'post-open' events for ons-sliding-menu view.
Added 'postchange', 'prechange' events for ons-tabbar.
Changed ons-sliding-menu's event names to 'preopen', 'postopen',
'preclose', 'postclose'.