Cannot save jams with random seeds

[bmcfee]

This is a regression introduced by #70 -- randomized deformers now have a random state field (deformer.rng) which is of type RandomState. This type is not JSON-serializable. When we go to save the output of a deformation (via JAMS), the encoder fails.

The easiest fix here is probably something like the following:

• Move all rng fields to _rng so that JAMS skips them during serialization
• Persist the random seeds in deformers as a separate field rng that is serialized. Raise a warning about serialization if the input is not serializable.

Since this breaks the object model a little bit, we'll need to bump the version number.