I didn't know nltk had a scipy dependency. As it apparently does, I'm not sure this should be in a `try`-`except`. As long as it is in a `try`-`except`, the corresponding `NameError` needs to be caught in `fisher` below (or, better explicit, set `fisher_exact` to `None` and test for it below).

(The [doc](http://nltk.org/install.html) still says `numpy` is an optional dependency, so I'm not certain `scipy` is required. @stevenbird?)