No, there isn't one that's available. Right now the GH SDK is not public for several reasons. The main three being that David's the only one who can document it, David's really the only one who can support it, and the SDK is still changing. There's far too much on David's plate as is, and if things wind up changing down the road, then putting the documentation together is all for not. Besides GH is still technically in the WIP stage of development, so until there's a full release (or at least a release date set), it would be hard to imagine an SDK being made available.