Fixing some argument bugs in pymc3.py.

The patch !293 (merged) introduced a bug which meant that a step_kwargs argument had to provided if there was a step argument, this patch fixes this by introducing an option if nuts_kwargs/step_kwargs is None:. It also allows the user to specify the draws argument in default_kwargs instead of the init (sorry for two patches!) @matthew-pitkin could you check if these are okay ?