add sharing current url by qrcode

use current url to generate qrcode,and show it in dropdown.
use https://github.com/davidshimjs/qrcodejs to generate qrcode

![2014-08-20_15-58-18](https://cloud.githubusercontent.com/assets/2015631/3978203/c7db9cae-2844-11e4-837f-05ef1ebc78d4.png)


it is disabled by default,set link.sharing.qrcode to true to use it.