Include regrade request information for submissions

[josephlou5]

When retrieving a submission using the actual GET endpoint https://api.codepost.io/submission/{id}, information about the regrade request associated with that submission is included. It would be nice if the SDK could also include that information, or provide some way to access it without making another manual request.

The additional keys would be:

questionIsOpen
questionText
questionDate
questionResponder
questionResponse
responseDate