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
I'm not sure what size limit we'd want to enforce.  Model size is going to depend on the content of the example essays (mostly # of unique words).

Even if we did set a limit, what's the appropriate response if the limit is exceeded?