Adds Developers Map in About dialog

Added a new public function  ```QgsApplication::developersMapFilePath()```
There is a check for internet connection in order to hide/show the developer map page into QStackedWidget
The leaflet framework read the json file found doc/contributors.json

![schermata del 2014-12-24 17 48 42](https://cloud.githubusercontent.com/assets/1374682/5549863/5b592e62-8b94-11e4-9957-a6afcfb98894.png)