Added push/pop support to the SMTLIB2 interface

Original patch from Matt Morehouse, but hand-applied because of
(whitespace?) divergences.

From 2f1873f846ce5bfe4731c27667c5122fc2d095e4 Mon Sep 17 00:00:00 2001
From: Matt Morehouse <mattmorehouse@gmail.com>
Date: Wed, 8 Oct 2014 10:09:36 -0500
Subject: [PATCH] Added push/pop support to the SMTLIB2 interface