Pack/Unpack implementation for Any.

We still need the JSON representation, which relies on something like a DescriptorPool to fetch message types from based on the type URL. That will come a bit later.
(The DescriptorPool comment in this commit is just a note which will prove useful if we use DescriptorPool itself.)
diff --git a/Makefile.am b/Makefile.am
index 82ce190..8b36178 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -95,6 +95,7 @@
   csharp/src/Google.Protobuf.Test/TestProtos/UnittestIssues.cs               \
   csharp/src/Google.Protobuf.Test/TestProtos/UnittestProto3.cs               \
   csharp/src/Google.Protobuf.Test/TestProtos/UnittestWellKnownTypes.cs       \
+  csharp/src/Google.Protobuf.Test/WellKnownTypes/AnyTest.cs                  \
   csharp/src/Google.Protobuf.Test/WellKnownTypes/DurationTest.cs             \
   csharp/src/Google.Protobuf.Test/WellKnownTypes/TimestampTest.cs            \
   csharp/src/Google.Protobuf.Test/WellKnownTypes/WrappersTest.cs             \
@@ -149,6 +150,7 @@
   csharp/src/Google.Protobuf/Reflection/ServiceDescriptor.cs                 \
   csharp/src/Google.Protobuf/Reflection/SingleFieldAccessor.cs               \
   csharp/src/Google.Protobuf/WellKnownTypes/Any.cs                           \
+  csharp/src/Google.Protobuf/WellKnownTypes/AnyPartial.cs                    \
   csharp/src/Google.Protobuf/WellKnownTypes/Api.cs                           \
   csharp/src/Google.Protobuf/WellKnownTypes/Duration.cs                      \
   csharp/src/Google.Protobuf/WellKnownTypes/DurationPartial.cs               \