Add SDImageCache removeImageForKey:fromDisk: to allow cache item removal from memory only (fix #111)

@@ -25,6 +25,7 @@
 - (void)queryDiskCacheForKey:(NSString *)key delegate:(id <SDImageCacheDelegate>)delegate userInfo:(NSDictionary *)info;
 
 - (void)removeImageForKey:(NSString *)key;
+- (void)removeImageForKey:(NSString *)key fromDisk:(BOOL)fromDisk;
 - (void)clearMemory;
 - (void)clearDisk;
 - (void)cleanDisk;

@@ -298,13 +298,22 @@ static SDImageCache *instance;
 
 - (void)removeImageForKey:(NSString *)key
 {
+    [self removeImageForKey:key fromDisk:YES];
+}
+
+- (void)removeImageForKey:(NSString *)key fromDisk:(BOOL)fromDisk
+{
     if (key == nil)
     {
         return;
     }
 
     [memCache removeObjectForKey:key];
-    [[NSFileManager defaultManager] removeItemAtPath:[self cachePathForKey:key] error:nil];
+
+    if (fromDisk)
+    {
+        [[NSFileManager defaultManager] removeItemAtPath:[self cachePathForKey:key] error:nil];
+    }
 }
 
 - (void)clearMemory