title: Warning raised by inserting 4-byte unicode to mysql
body: <p>Look at the following:</p>
  
  <pre><code>/home/kinka/workspace/py/tutorial/tutorial/pipelines.py:33: Warning: Incorrect string 
  value: '\xF0\x9F\x91\x8A\xF0\x9F...' for column 't_content' at row 1
  n = self.cursor.execute(self.sql, (item['topic'], item['url'], item['content']))
  </code></pre>
  
  <p>The string <code>'\xF0\x9F\x91\x8A</code>, actually is a 4-byte unicode: <code>u'\U0001f62a'</code>. The mysql's character-set is utf-8 but inserting 4-byte unicode it will truncate the inserted string.
  I googled for such a problem and found that mysql under 5.5.3 don't support 4-byte unicode, and unfortunately mine is 5.5.224.
  I don't want to upgrade the mysql server, so I just want to filter the 4-byte unicode in python, I tried to use regular expression but failed.
  So, any help?</p>