Adds Rake task for compiling protobuf files

[RJPercival]

Adds "rake/generate_task" which can be used in a Rakefile as follows:

Protobuf::GenerateTask.new("proto/*.proto") do |ruby_protobuf|
  spec.files << ruby_protobuf
end

This adds a "protobuf" task, which can be used by executing "rake protobuf". If used as above, it will also run automatically whenever a packaging task occurs, e.g. "package", "build" or "gem". It can also manually be added as a dependency of any other tasks, e.g. "test".