Functionality for purging a single image from the cache.

For now, this image is selected at random, but a subtler method --
e.g. selecting the least-frequently-used image, or the image that
whose last-requested date is earliest -- should be developed soon.

expurgate.php

@@ -159,7 +159,12 @@ function($size, $cache) {
 // Removes one image from the cache. This means that, when the cache is full,
 // we'll be adopting a one in, one out policy
 function purge_cache() {
+	$cache_files = get_cache_files();
+
+	// For now, select a file at random.
+	$to_delete = $cache_files[array_rand($cache_files)];
 
+	unlink(CACHE_DIR . '/' . $to_delete->filename);
 }
 
 // Removes expired entries from the cache.