Pull Request for placeholder feature of TITokenField 

Hi Tom,

Thank you for your help on Twitter today. I have implemented the placeholder feature for TITokenField and wondering if you would like to pull to your repository. I have never do a patch and sent out pull request before. so if you need me to do more things or refactoring the code, please let me know. 

Thank you again for this project and your help. Really appreciate it. 

Sincerely,
Zhao