Update GameServiceContainer.cs

Added generic overloads for AddService() and GetService().
Simpler and cleaner to use, and with the generic GetService() there's also need to cast/unbox from object.
Update GamerServiceContainer.cs - removed exception

Ok, so we return null instead of throwing the exception... and in order to achieve this T must be a reference type.
Merge pull request #2566 from TommasoScalici/develop

Update GameServiceContainer.cs