Add Dropdown menu and Facebook like slide-in menu

In addition to #17 , Facebook like slide-in menu.

notice.
- In application_controller, I added mix-in UserDecorator to 'current_user', since I could not use 'show_name' method. 
- I used Sidr for slide-in menu.
- I bind sidr('close') to menu's click/touch event, becouse it didn't work correctly.  

@adorechic Please review it.