text: Title: TweetTokenizer fails to remove user handles in text.
  Body: The below input should remove all user handles which start with "@".
  
  input: @remy:This is waaaaayyyy too much for you!!!!!!@adam 
  output : [':', 'This', 'is', 'waaayyy', 'too', 'much', 'for', 'you', '!', '!', '!', '@adam']
  
  The TweetTokenizer fail to remove the user handle of Adam.
  
  I would like to open a pull request that solves the following issues:- 
  
  - Improve the regular expression pattern to detect in-text handles properly.
  - Adjust the length of characters to be 15 instead of 20. 