Fixed  #442 added app.getDataPath()

I've added an app.getDataPath function that returns the data path for the OS like node-webkit's app.dataPath.

 * `%APPDATA%\My App Name` (`C:\Users\[username]\AppData\Roaming\My App Name`) in Windows
 * `~/.config/My App Name` in Linux
 * etc

Because I don't have access to a Mac I haven't been able to test this on OSX
You can include the `atom/common/native_mate_converters/file_path_converter.h` header and just returns `base::FilePath` here.