Run Python unittests and doctests.

Add a meta/check.sh script that runs unit tests before the syntax
linters.

Add unittest converters for the existing doctests.

meta/check.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+set -e
+cd $(dirname "$0")
+
+# Run unit tests.
+python -m unittest discover
+
+# Check Python sources for Python 3 compatibility using pylint.
+#
+# Install pylint with 'pip install pylint'.
+pylint --py3k --reports=no -- *.py cretonne isa
+flake8 .