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
Actually, it looks like it makes more sense to use `_dump` and `self._load`.

```ruby
def _dump
  [to_hash, api_key]
end

def self._load(args)
  hash, api_key = args
  instance = new(hash["id"], api_key)
  instance.refresh_from(hash, api_key)
  instance
end
```

Eh, don't really care.