More reasonable support Artist ID

Before(Both iPhone/iTouch and iPad):
-  iPhone/iTouch Device
    - If one Artist ID **only has iPhone apps**, the url request doesn't return JSON data.   **X**
    - If one Artist ID **only has iPad apps**, the url request doesn't return JSON data.   **X**
    - If one Artist ID has **iPhone and iPad apps**, the url request return iPhone and iPad apps' JSON data.   **√ but not  very accurate I think. (iPhone will get iPad only apps information...)**

- iPad Device
    - If one Artist ID **only has iPhone apps**, the url request doesn't return JSON data.   **X**
    - If one Artist ID **only has iPad apps**,  the url request doesn't return JSON data.   **X**
    - If one Artist ID has **iPhone and iPad apps**, the url request return iPhone and iPad apps' JSON data.   **√ but not  very accurate I think. (iPad will get iPhone only apps information...)**


Now:
- iPhone/iTouch Device
    - If one Artist ID **only has iPhone apps**, the url request return iPhone apps' JSON data.  **√**
    - If one Artist ID **only has iPad apps**, the url request doesn't return JSON data, I think it's ok for a iPhone/iTouch user   **√**
    - If one Artist ID has **iPhone and iPad apps**,  the url request return iPhone apps' JSON data. **√**

- iPad Device
    - If one Artist ID **only has iPhone apps**, the url request doesn't return iPhone apps' JSON data,  I think it's ok for a iPad user **√**
    - If one Artist ID **only has iPad apps**, the url request return JSON data.  **√**
    - If one Artist ID  **has iPhone and iPad apps**,  the url request return iPad apps' JSON data.   **√**

If developer want to show all apps of the Artist ID , the API is:
```OBJC
- (void)loadAllAppsWithArtistId:(NSInteger)artistId
                completionBlock:(void(^)(BOOL result, NSError *error))block;
```