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...
diff --git a/csharp/generate_protos.sh b/csharp/generate_protos.sh
index 0d217a9..3a556b0 100755
--- a/csharp/generate_protos.sh
+++ b/csharp/generate_protos.sh
@@ -35,11 +35,10 @@
   fi
 fi
 
-# Descriptor proto
-$PROTOC -Isrc --csharp_out=csharp/src/Google.Protobuf/Reflection \
-    src/google/protobuf/descriptor.proto
-
-$PROTOC -Isrc --csharp_out=csharp/src/Google.Protobuf/WellKnownTypes \
+# descriptor.proto and well-known types
+$PROTOC -Isrc --csharp_out=csharp/src/Google.Protobuf \
+    --csharp_opt=base_namespace=Google.Protobuf \
+    src/google/protobuf/descriptor.proto \
     src/google/protobuf/any.proto \
     src/google/protobuf/api.proto \
     src/google/protobuf/duration.proto \
@@ -51,15 +50,18 @@
     src/google/protobuf/type.proto \
     src/google/protobuf/wrappers.proto
 
-$PROTOC -Isrc --csharp_out=csharp/src/Google.Protobuf.Test/TestProtos \
+# Test protos where the namespace matches the target location
+$PROTOC -Isrc --csharp_out=csharp/src/Google.Protobuf.Test \
+    --csharp_opt=base_namespace=Google.Protobuf \
     src/google/protobuf/map_unittest_proto3.proto \
     src/google/protobuf/unittest_proto3.proto \
     src/google/protobuf/unittest_import_proto3.proto \
     src/google/protobuf/unittest_import_public_proto3.proto \
     src/google/protobuf/unittest_well_known_types.proto
 
-
-$PROTOC -Icsharp/protos --csharp_out=csharp/src/Google.Protobuf.Test/TestProtos \
+# Different base namespace to the protos above
+$PROTOC -Icsharp/protos --csharp_out=csharp/src/Google.Protobuf.Test \
+    --csharp_opt=base_namespace=UnitTest.Issues \
     csharp/protos/unittest_issues.proto
 
 # AddressBook sample protos