#437 add a popup with QR Code encoded for a topic

Every topic then get a button to popup a dialog with QR Code inside.
Then, user can scan it and open the topic in a webview in wechat or some other tools in mobile phone.
# Get a Button
![screen shot 2014-10-26 at 21 19 29](https://cloud.githubusercontent.com/assets/3538629/4783008/ddde7d56-5d13-11e4-87ac-32a8eba6ffc5.png)

# Get the QR Code
![screen shot 2014-10-26 at 21 19 20](https://cloud.githubusercontent.com/assets/3538629/4783009/efefb708-5d13-11e4-84ac-2cabb58bdc5d.png)