Add QRCode on shared folder public link

Add an option on action.share plugin to create QRCode when using shares with public link.
![share_qrcode](https://f.cloud.github.com/assets/2251352/1377157/f11ce726-3aa1-11e3-9f27-63e455d18125.png)

It uses qrcode.js (http://davidshimjs.github.io/qrcodejs/ version) under MIT license. I'm curious to know if this pull request match CLA?