This change only adds basic proto2 support without advanced features
like extensions, custom options, maps, etc.

The protoc binary now generates ruby code for proto2 syntax.
However, for now, it is restricted to proto2 files without advanced features
like extensions, in which case it still errors out.

This change also modifies the DSL to add proto messages to the DescriptorPool.
There is a new DSL Builder#add_file to create a new FileDescriptor. With this,
the generated ruby DSL looks something like:

Google::Protobuf::DescriptorPool.generated_pool.build do
  add_file "test.proto" do
    add_message "foo" do
      optional :val, :int32, 1
    end
  end
end