I found this paper useful for a side project I worked on a few months ago, one that made use of n-grams in a naive bayesian classifier:

http://arxiv.org/pdf/1305.6143v2.pdf

and the lead authors's github repos are:

https://github.com/vivekn/sentiment https://github.com/vivekn/sentiment-web

He's implemented 'negative bi-gram detection' (my phrasing, not his) with this function:

https://github.com/vivekn/sentiment/blob/master/info.py#L26-...

...which I found useful as a jumping off point. Good luck!