Check if this credential has already been added to the database, if not add it in.

def add_credential(self, url, username, password):
        """
        Check if this credential has already been added to the database, if not add it in.
        """
        cur = self.conn.cursor()

        cur.execute("SELECT * FROM credentials WHERE LOWER(username)=LOWER(?) AND password=?", [url, username, password])
        results = cur.fetchall()

        if not len(results):
            cur.execute("INSERT INTO credentials (username, password) VALUES (?,?)", [username, password] )

        cur.close()