update podspec

YYImage.podspec

@@ -11,13 +11,17 @@ Pod::Spec.new do |s|
   s.source       = { :git => 'https://github.com/ibireme/YYImage.git', :tag => s.version.to_s }
   
   s.requires_arc = true
-  s.source_files = 'YYImage/*.{h,m}'
-  s.public_header_files = 'YYImage/*.{h}'
   
   s.libraries = 'z'
   s.frameworks = 'UIKit', 'CoreFoundation', 'QuartzCore', 'AssetsLibrary', 'ImageIO', 'Accelerate', 'MobileCoreServices'
+
+  s.subspec 'Core' do |core|
+    core.source_files = 'YYImage/*.{h,m}'
+    core.public_header_files = 'YYImage/*.{h}'
+  end
   
   s.subspec 'WebP' do |webp|
+    webp.dependency 'YYImage/Core'
     webp.ios.vendored_frameworks = 'Vendor/WebP.framework'
   end