Use cPickle instead of pickle; add additional tests

`cPickle` is the (much) faster version of the `pickle` module.  I've also added some additional tests from the `will/scikit-classifier` branch, in particular for testing that this plays nicely with JSON serialization.

@stephensanchez
Do pickle and cPickle serialize and deserialize the same way? 

I'm wondering about a scenario were someone does not have cPickle in their environment, runs some tasks, then updates their environment.