Adds Users Map in About dialog

Added a new public function  ```QgsApplication::userMapFilePath()```
There is a check for internet connection in order to hide/show the user map page into QStackedWidget
The leaflet framework read the json file found doc/contributors.json

![schermata del 2014-10-18 16 48 09](https://cloud.githubusercontent.com/assets/1374682/4689974/816bc4fa-56d5-11e4-82da-eef71a909b01.png)