Support random.get_state(), random.set_state().

It would be nice to support the `numpy.random` methods `get_state()` and `set_state()` for the distributed system. This would let the user save and restore the global prng generator state, even when they did not use a deterministic seed to begin with.

For the distributed system, this would presumably work with a list of generator states, one per engine, since these are in general different. I started to try an implement this in PR #253, but the data being communicated is fairly large (about 624 integers per engine), which was slow. This specific usage was considered less important than what #253 provides, so this issue is created to note the desire for later.