Add coveragerc, this simplifies testing

.coveragerc

@@ -0,0 +1,6 @@
+[run]
+omit = pwman/tests/*.py, pwman/ui/mac.py, pwman/ui/win.py, \
+       pwman/data/convertdb.py, pwman/data/nodes_multiple_approach.py, \
+       pwman/data/models.py, pwman/data/drivers/postgresql.py, \
+       pwman/data/drivers/mysql.py, pwman/exchange/importer.py, pwman/ui/base.py
+source = pwman