generate HTML coverage reports for unit tests

.gitignore

@@ -2,3 +2,4 @@
 /.waf*
 /build/
 /test/tmp/
+/coverage/

Makefile

@@ -9,3 +9,5 @@ clean:
 test:
 	./waf build --targets tests
 	./build/tests
+	mkdir -p coverage
+	-(cd build; gcovr --object-directory=src/core --root=.. -o ../coverage/index.html --html --html-details --gcov-filter='.*src#core.*')