Code Generator produces messages with broken dependencies

This is part of the output of running the generator from 67f4894e4a77bac6e3b5953ca51f2de12127fff7 against https://github.com/basho/riak_pb/blob/master/src/riak_kv.proto :
```ruby

class RpbContent
  include Beefcake::Message


  required :value, :bytes, 1
  optional :content_type, :bytes, 2
  optional :charset, :bytes, 3
  optional :content_encoding, :bytes, 4
  optional :vtag, :bytes, 5
  repeated :links, RpbLink, 6
  optional :last_mod, :uint32, 7
  optional :last_mod_usecs, :uint32, 8
  repeated :usermeta, RpbPair, 9
  repeated :indexes, RpbPair, 10
  optional :deleted, :bool, 11

end

class RpbLink
  include Beefcake::Message


  optional :bucket, :bytes, 1
  optional :key, :bytes, 2
  optional :tag, :bytes, 3

end
```
The issue is that `RpbContent.links` cannot be defined, because it depends on the `RpbLink` class that hasn't been read yet.

What I'll try and do this week is find a way to create all the classes up front, so they all exist when they try to get tagged in messages.