Add Google Protobuf support with a MessageConverter

This change adds a new HttpMessageConverter supporting
Google protocol buffers (aka Protobuf).
This message converter supports the following media types:
* application/json
* application/xml
* text/plain
* text/html (output only)
* and by default application/x-protobuf

Note, in order to generate Proto Message classes, the protoc binary
must be available on your system.

Issue: SPR-5807/SPR-6259

spring-web/src/test/proto/sample.proto

@@ -0,0 +1,12 @@
+option java_package = "org.springframework.protobuf";
+option java_outer_classname = "OuterSample";
+option java_multiple_files = true;
+
+message Msg {
+    optional string foo = 1;
+    optional SecondMsg blah = 2;
+}
+
+message SecondMsg {
+    optional int32 blah = 1;
+}