[Developers] Update calling code property of Country

[Bryan Cheung]

https://bugs.freebase.com/browse/DA-749

The property /location/country/calling code should be changed from  
expecting a machine-readable string to expecting an integer.  Some  
instances include "+"; this symbol should simply be dropped.  This  
shouldn't affect queries, but if this is going to be an issue with  
anyone, please speak up.

Bryan