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.

Removed the now-un-needed `-registerDeviceToken:withAlias:success:failure` from Urban airship subclass and from Parse subclass.

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 and tags for personal use