Add coverage.py invocation to Makefile

Signed-off-by: martin f. krafft <madduck@madduck.net>
diff --git a/Makefile b/Makefile
index f8c756c..0f7eb77 100644
--- a/Makefile
+++ b/Makefile
@@ -18,3 +18,9 @@
 lint-report: ARGS=--report=y
 lint-report: lint
 .PHONY: lint-report
+
+coverage: .coverage
+	python-coverage -r -m
+.PHONY: coverage
+.coverage: $(PYFILES)
+	python-coverage -x setup.py nosetests