Adapt to ios8

Before ios8 , the value of `[UIScreen mainScreen].bounds` takes no consideration to device orientation , so if we want to get the **correct rect** , which is based on the current device orientation we should do as follow :

    - (CGRect)currentScreenBoundsDependOnOrientation
    {  
           CGRect screenBounds = [UIScreen mainScreen].bounds ;
          CGFloat width = CGRectGetWidth(screenBounds)  ;
          CGFloat height = CGRectGetHeight(screenBounds) ;
          UIInterfaceOrientation interfaceOrientation = [UIApplication sharedApplication].statusBarOrientation;
          if(UIInterfaceOrientationIsPortrait(interfaceOrientation)){
               screenBounds.size = CGSizeMake(width, height);
          }else if(UIInterfaceOrientationIsLandscape(interfaceOrientation)){
               screenBounds.size = CGSizeMake(height, width);
          }
          return screenBounds ;
     }
this method works well before ios8.

But ,after ios8.0 , `[UIScreen mainScreen].bounds` now takes consideration of the device rotation as the [reference](https://developer.apple.com/library/ios/documentation/UIKit/Reference/UIScreen_Class/#//apple_ref/occ/instp/UIScreen/bounds) show :

>Discussion
>--------------------
This rectangle is specified in the current coordinate space, which takes into account any interface rotations in effect for the device. Therefore, the value of this property may change when the device rotates between portrait and landscape orientations.

so the solve this compatibility problem we should do as follow

     #define IsIOS8 (NSFoundationVersionNumber > NSFoundationVersionNumber_iOS_7_1)
    -(CGRect)currentScreenBoundsDependOnOrientation
    {  
          CGRect screenBounds = [UIScreen mainScreen].bounds ;
          if(IsIOS8){
             return screenBounds ;
         }
         CGFloat width = CGRectGetWidth(screenBounds)  ;
         CGFloat height = CGRectGetHeight(screenBounds) ;
         UIInterfaceOrientation interfaceOrientation = [UIApplication sharedApplication].statusBarOrientation;
         if(UIInterfaceOrientationIsPortrait(interfaceOrientation)){
               screenBounds.size = CGSizeMake(width, height);
         }else if(UIInterfaceOrientationIsLandscape(interfaceOrientation)){
              screenBounds.size = CGSizeMake(height, width);
        }
        return screenBounds ;
     }
Refer to the [stackoverflow](http://stackoverflow.com/questions/7905432/how-to-get-orientation-dependent-height-and-width-of-the-screen/13331743#13331743) of this problem.