Rename Message#fields

I'm implementing a client library for Mozilla Heka, and their [message.proto](https://github.com/mozilla-services/heka/blob/master/message/message.proto) contains a `fields` field to embed more `Field` message type. Although I don't agree with their decision, the ruby client should support that format. 

When the `Message` message type compiled by `ruby-protocol-buffers` tries to initiate itself with the `fields` field in it, runs into an infinite loop. I know, the whole thing seems ridiculous.

My first proposal is to add a `message_` prefix to `fields`. If you have a better naming convention, I'll change this.