Add wrapper for OTAPI_WrapMessage_GetSuccess()

Still need to figure out what specific verification happens here.

Should be used by easy.RegisterNym() now.
 Otto Allmendinger wrote:
>> +	retval := opentxs.OTAPI_WrapMessage_GetSuccess(message)
>
> `response` suggest network activity. What about `result`?
>

In this case aren't we taking a reply from the server, and calling this
function in order to see what the response is?