Implemented #marshal_load and #marshal_dump for EasyPost::EasyPostObject

This allows serializing `EasyPost` objects using `Marshal`. I needed this for `Rails.cache` in a jiffy, but feel free to pull and reuse. Related to issue https://github.com/EasyPost/easypost-ruby/issues/4
This should be written:

```
def marshal_load(args)
  hash, api_key = args
  initialize(hash["id"], api_key)
  refresh_from(hash, api_key)
end
```

But, maybe YAML or JSON serialization would be better?