Adding Kneser-Ney Probability Distribution

Hello!

I noticed that while the NLTK probability module does contain most common probability distributions, it has none representing the Kneser-Ney technique. 
As a final project for a class I took a stab at creating one and was wondering if there would be any interest in incorporating it into the main branch of the NLTK project.  I currently coded up only one of the versions of this algorithm, but if there is demand and interest for more, I'm willing to write up some other versions as well.

Please feel free to let me know where you think my code or comments need improvements, I will gladly use this opportunity to learn something new about the toolkit and/or python.

Cheers!
Ilia.