Added API for Retrieving Credit History

Also, while testing, I don't actually see the message in the logs. There's an 'error' object now that contains the message.

2014-10-11 13:50:35.677 Branch-TestBed[28513:1891034] <0x17000a9f0 ServerInterface.m:(114)> returned = Tag: t_get_reward_history; Status: 404; Data: {
    error =     {
        code = 404;
        message = "The resource you were looking for could not be found.";
    };
}
2014-10-11 13:50:35.678 Branch-TestBed[28513:1891034] Branch API Error: (null)