[Qt] Optionally add third party links to transaction context menu

Adds the possibility to open a transaction in block explorer through right-click context menu in transactions tab.

![thirdpartyurls1](https://cloud.githubusercontent.com/assets/2814559/2796411/0f7344c4-cc0e-11e3-9492-d427ece261d7.png)

       https://blockchain.info/tx/%s;https://blockexplorer.com/tx/%s;http://luckyb.it/browse?search=%s


![thirdpartyurls3](https://cloud.githubusercontent.com/assets/2814559/2796414/1a786a16-cc0e-11e3-9787-6145d0f64002.png)


Can also be setup on testnet, as testnet uses a different qt-settings file.
To make the implementation simple, changing the option needs client restart to take effect.