Use Z3_model_eval instead of the deprecated Z3_eval.

For BC we set reconstruct_models to false, but it might be
avantageous to turn it on and let Z3 complete partial models instead of
doing it ourselves later (i.e. in Leon).
Separate model_eval and eval, use model_eval in Z3Model.eval
Merge pull request #22 from colder/topic/eval

Use Z3_model_eval instead of the deprecated Z3_eval.