Add NSCache countLimit property in attempts to get better control of cache eviction.

SDWebImage/SDImageCache.h

@@ -47,6 +47,11 @@ typedef void(^SDWebImageCalculateSizeBlock)(NSUInteger fileCount, NSUInteger tot
  */
 @property (assign, nonatomic) NSUInteger maxMemoryCost;
 
+/**
+ * The maximum number of objects the cache should hold.
+ */
+@property (assign, nonatomic) NSUInteger maxMemoryCountLimit;
+
 /**
  * The maximum length of time to keep an image in the cache, in seconds
  */

SDWebImage/SDImageCache.m

@@ -377,6 +377,14 @@ FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
     return self.memCache.totalCostLimit;
 }
 
+- (NSUInteger)maxMemoryCountLimit {
+    return self.memCache.countLimit;
+}
+
+- (void)setMaxMemoryCountLimit:(NSUInteger)maxCountLimit {
+    self.memCache.countLimit = maxCountLimit;
+}
+
 - (void)clearMemory {
     [self.memCache removeAllObjects];
 }