Give a proxy object that can be used as a generic serializer

I want to use EJSON as a serializer in an ActiveRecord model.

Unfortunately EJSON do not respect the ruby Serializer convention, so I needed to add a proxy object to not break the API :/.

Example usage

```ruby
class User
  serialize credentials, EJSON.new(pubkey, privkey).serializer
end
```

/review @burke @manygrams