fix issue#24

[schmargi]

Fix #24

Now, when the user has two layers like:

p_t_red-button AND p_m_red-button and export them.

In folder "Images.xcassets/red-button.imageset/" There are now the images:

• ipad_red-button.png
• ipad_red-button@2x.png
• ipad_red-button@3x.png
• iphone_red-button.png
• iphone_red-button@2x.png
• iphone_red-button@3x.png

and also the Content.json for both

{  
   "images":[  
      {  
         "idiom":"ipad",
         "scale":"1x",
         "filename":"ipad_red-button.png"
      },
      {  
         "idiom":"ipad",
         "scale":"2x",
         "filename":"ipad_red-button@2x.png"
      },
      {  
         "idiom":"ipad",
         "scale":"3x",
         "filename":"ipad_red-button@3x.png"
      },
      {  
         "idiom":"universal",
         "scale":"1x",
         "filename":"universal_red-button.png"
      },
      {  
         "idiom":"universal",
         "scale":"2x",
         "filename":"universal_red-button@2x.png"
      },
      {  
         "idiom":"universal",
         "scale":"3x",
         "filename":"universal_red-button@3x.png"
      }
   ],
   "info":{  
      "version":1,
      "author":"xcode"
   }
}