Created payload property, refactored to take advantage

Moved default payloads to new "configureDefaultPayload" methods.
This simplifies subclasses, and allows users of the default Orbiter class to tweak the payload without losing all the defaults.

e.g.

    Orbiter *orbiter = [[Orbiter alloc] initWithBaseURL:serverURL credential:nil];
    [orbiter.payload setObject:@"tlh" forKey:@"language"];
    [orbiter registerDeviceToken:deviceToken withPayload:extras success:^(id responseObject) {
        NSLog(@"Registration Success: %@", responseObject);
    } failure:^(NSError *error) {
        NSLog(@"Registration Error: %@", error);
    }];

	// still includes all the default values for tags, timezone, etc.
updated podspec, added tag (for personal use)