add coverage makefile target

Requires the coverage python package.
diff --git a/.gitignore b/.gitignore
index c0b10be..f4243d4 100644
--- a/.gitignore
+++ b/.gitignore
@@ -12,3 +12,5 @@
 *.cache
 openflowj-loxi
 /bin
+*,cover
+/.coverage