Add the ability to set max cache age.

SDWebImage/SDImageCache.h

@@ -27,6 +27,13 @@
  */
 + (SDImageCache *)sharedImageCache;
 
+/**
+ * Sets the global maximum cache age
+ *
+ * @param maxCacheAge The maximum length of time to keep an image in the cache, in seconds
+ */
++ (void) setMaxCacheAge:(NSInteger) maxCacheAge;
+
 /**
  * Store an image into memory and disk cache at the given key.
  *

SDWebImage/SDImageCache.m

@@ -123,6 +123,11 @@ static natural_t get_free_memory(void)
     return instance;
 }
 
++ (void) setMaxCacheAge:(NSInteger)maxCacheAge
+{
+    cacheMaxCacheAge = maxCacheAge;
+}
+
 #pragma mark SDImageCache (private)
 
 - (NSString *)cachePathForKey:(NSString *)key