Adding PennTreeBank Tokenizer

Hello, 

I have a NLP project in PHP. I want to combine our efforts into building a complete suite of NLP tools in PHP. My github project can be found at https://github.com/yooper/text-analysis. I will gladly contribute to your project since your project is much more mature. This is one of many pull requests I can put in to further expand the functionality and feature richness of the PHP NLP Tools project.