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
The binary output is compatible; it's just implemented differently (one is native C, the other is Python).  This is the recommended way to do the import, but our target environment (Ubuntu) will have cPickle installed.