Issue #110: Updates background fetch API

Background Fetch API has been updated. Usage should be as follows:

```
- (void)application:(UIApplication *)application performFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler
{
    [self.simperium backgroundFetchWithCompletion:^(BOOL syncedNewData) {

        UIBackgroundFetchResult result = syncedNewData ? UIBackgroundFetchResultNewData : UIBackgroundFetchResultNoData;
        completionHandler(result);
    }];
}
```