Added a keyExists method to CCCache

Author: Birkemose

cocos2d/CCCache.h

@@ -45,6 +45,7 @@
 - (instancetype)init;
 
 - (void)preload:(id<NSCopying>)key;
+- (BOOL)keyExists:(id<NSCopying>)key;
 
 - (id)rawObjectForKey:(id<NSCopying>)key;
 - (id)objectForKey:(id<NSCopying>)key;

cocos2d/CCCache.m

@@ -115,6 +115,11 @@ - (void)makeAlias:(id<NSCopying>)alias forKey:(id<NSCopying>)key
 		[_cacheList setObject:entry forKey:alias];
 }
 
+- (BOOL)keyExists:(id<NSCopying>)key
+{
+    return([_cacheList objectForKey:key] != nil);
+}
+
 //------------------------------------------------------------------------------
 
 - (void)flush