crawler: improve sqlite back end

Autocommit & fixed a bug

morss/crawler.py

@@ -482,7 +482,7 @@ import sqlite3
 
 class SQLiteCache(BaseCache):
     def __init__(self, filename=':memory:'):
-        self.con = sqlite3.connect(filename or sqlite_default, detect_types=sqlite3.PARSE_DECLTYPES, check_same_thread=False)
+        self.con = sqlite3.connect(filename, detect_types=sqlite3.PARSE_DECLTYPES, check_same_thread=False, isolation_level=None)
 
         with self.con:
             self.con.execute('CREATE TABLE IF NOT EXISTS data (url UNICODE PRIMARY KEY, code INT, msg UNICODE, headers UNICODE, data BLOB, timestamp INT)')