Fix up some problems with the ReviewReply model

There were a couple issues with RB.ReviewReply from my previous implementation.
First, I had used underscores in the property names instead of using camelCase.
This involved some changes to the plumbing but is less than I had expected.
Second, the _.defaults call wasn't passing in the correct prototype defaults, so
the resulting object didn't have some of the right attributes.

Replied to a review with diff comments, body_top and body_bottom fields.