goprotobuf: Present comments in .proto files in the generated .pb.go.

This doesn't use every comment, but just the ones that I thought
would be most useful to someone reading the generated code, namely
those attached to these things:
        - messages
        - message fields
        - enums
        - enum values

R=r
CC=golang-dev
https://codereview.appspot.com/11889044
7 files changed