Generate a directory hierarchy based on namespace (C#)

This introduces a new C# option, base_namespace.
If the option is not specified, the behaviour is as before: no directories are generated.
If the option *is* specified, all C# namespaces must be relative to the base namespace, and the directories are generated relative to that namespace.

Example:
- Any.proto declares csharp_namespace = "Google.Protobuf.WellKnownTypes"
- We build with --csharp_out=Google.Protobuf --csharp_opt=base_namespace=Google.Protobuf
- The Any.cs file is generated in Google.Protobuf/WellKnownTypes (where it currently lives)

We need a change to descriptor.proto before this will all work (it wasn't in the right C# namespace) but that needs the other descriptors to be regenerated too. See next commit...