Coverage tracking for the test suite.

If you issue `make test`, it will run the tests and print out a coverage
reporting indicating what lines of code were covered by the tests and which
were not.  This will (hopefully) be useful more over time as we start to write
more tests.

Makefile

@@ -63,6 +63,9 @@ clean:
 git-clean:
 	@git clean -d -q -x
 
+test:
+	nosetests --with-coverage --cover-package .
+
 subdirs:
 	for d in $(SUBDIRS); do make -C $$d; [ $$? = 0 ] || exit 1; done