Add coverage to Makefile and improve clean target

Makefile

@@ -7,6 +7,11 @@ clean:
 	rm -rf build
 	rm -rf dist
 	rm -rf scour.egg-info
+	rm -rf .tox
+	rm -f .coverage*
+	rm -rf htmlcov
+	find . -name "*.pyc" -type f -exec rm -f {} \;
+	find . -name "*__pycache__" -type d -prune -exec rm -rf {} \;
 
 publish: clean
 	python setup.py register
@@ -26,3 +31,8 @@ test_error_on_flowtext:
 
 flake8:
 	flake8 --max-line-length=119
+
+coverage:
+	coverage run --source=scour testscour.py
+	coverage html
+	coverage report