add a makefile I use for developing pasn1.

It uses entr to watch for edits, and then automatically runs the tests
and prints out code coverage.

Makefile

@@ -0,0 +1,4 @@
+MODULES=pasn1.py
+
+test:
+	(echo $(MODULES) | entr sh -c 'python -m coverage run -m unittest $(basename $(MODULES)) && coverage report -m')