Login/Logout durin in-app Payments

This is a test for  https://github.com/mozilla/marketplace-tests-gaia/issues/61
See https://bugzilla.mozilla.org/show_bug.cgi?id=1103072 for more details

The test currently installs an app from resources and launches it. This needs more modifications.
When running the test command line needs <code>--server-root=marketplacetests/resources/webapps/packaged1</code>