Set version to 4.0.0 and updated CHANGELOG

+## [4.0.0 - New platforms (Mac OS X and watchOS) + refactoring, on Jan 28th, 2017](https://github.com/rs/SDWebImage/releases/tag/4.0.0)
+
+See [all tickets marked for the 4.0.0 release](https://github.com/rs/SDWebImage/milestone/3)
+Versions 4.0.0-beta and 4.0.0-beta 2 list all the changes. 
+
 ## [4.0.0 beta 2 - New platforms (Mac OS X and watchOS) + refactoring, on Oct 6th, 2016](https://github.com/rs/SDWebImage/releases/tag/4.0.0-beta2)
 
 See [all tickets marked for the 4.0.0 release](https://github.com/rs/SDWebImage/milestone/3)

 Pod::Spec.new do |s|
   s.name = 'SDWebImage'
-  s.version = '4.0.0-beta2'
+  s.version = '4.0.0'
 
   s.osx.deployment_target = '10.8'
   s.ios.deployment_target = '7.0'

@@ -15,7 +15,7 @@
 	<key>CFBundlePackageType</key>
 	<string>FMWK</string>
 	<key>CFBundleShortVersionString</key>
-	<string>4.0.0-beta2</string>
+	<string>4.0.0</string>
 	<key>CFBundleSignature</key>
 	<string>????</string>
 	<key>CFBundleVersion</key>