Fixes marshaling of Stripe Objects

Trying to marshal an object in ruby that has opened the singleton class gives you the following:

    TypeError: singleton can't be dumped
        from (irb):5:in `dump'
        from (irb):5
        from /Users/andrewpthorp/.rbenv/versions/1.9.3/bin/irb:12:in `<main>'

The solution is to add `#marshal_dump` and `#marshal_load` to the class, so the singleton class is never attempted to be marshaled.

Fixes #90