goprotobuf: Introduce UnmarshalAppend.
goprotobuf: Introduce UnmarshalAppend.

R=r
CC=golang-dev
http://codereview.appspot.com/6347058

jmptrader/protobuf-1,francoishill/protobuf,csuideal/protobuf,aoeu/protobuf,velocity-toolkit/protobuf-1,tamird/protobuf-2,trdata/protobuf,ekle/protoc-gen-goweb_test,daemonl/protobuf,golang/protobuf,tamird/protobuf-2,alskjstl/protobuf,hailocab/protobuf,adnaan/protobuf,eduardiazf/protobuf,jrbarron/protobuf,trdata/protobuf,mwitkow-io/protobuf,ArtemTeleshev/go-protobuf,kung-foo/protobuf,skyportsystems/go-protobuf,skyportsystems/go-protobuf,golang/protobuf,jeremyletang/protobuf,fuchsia-mirror/third_party-github.com-golang-protobuf,linkedin-inc/protobuf,qinlodestar/protobuf,ekle/protoc-gen-goweb,fuchsia-mirror/third_party-github.com-golang-protobuf