fixing nasty deletion bug from #15, included unit tests to trigger it

and reworked persistence classes to through exceptions rather to fail
silently

lib/zerobin/db.php

@@ -80,13 +80,6 @@ class zerobin_db extends zerobin_abstract
                 array_key_exists('opt', $options)
             )
             {
-                self::$_db = new PDO(
-                    $options['dsn'],
-                    $options['usr'],
-                    $options['pwd'],
-                    $options['opt']
-                );
-
                 // check if the database contains the required tables
                 self::$_type = strtolower(
                     substr($options['dsn'], 0, strpos($options['dsn'], ':'))
@@ -132,9 +125,16 @@ class zerobin_db extends zerobin_abstract
                         throw new Exception(
                             'PDO type ' .
                             self::$_type .
-                            ' is currently not supported.'
+                            ' is currently not supported.',
+                            5
                         );
                 }
+                self::$_db = new PDO(
+                    $options['dsn'],
+                    $options['usr'],
+                    $options['pwd'],
+                    $options['opt']
+                );
                 $statement = self::$_db->query($sql);
                 $tables = $statement->fetchAll(PDO::FETCH_COLUMN, 0);
 
@@ -266,7 +266,7 @@ class zerobin_db extends zerobin_abstract
             array($pasteid)
         );
         if (
-        		array_key_exists($pasteid, self::$_cache)
+            array_key_exists($pasteid, self::$_cache)
         ) unset(self::$_cache[$pasteid]);
     }