Bug 989562 - Write a test to install & launch a packaged app

Sorry, I had to remove these commits, because I accidentally put them on master (I still have them now in a separate branch 989562_v3.

I have now a completely different PR that's not using marketplace at all, but I still wanted to comment on the review requests.

I added this: _app_root = (By.CSS_SELECTOR, ':root')
        self.wait_for_element_displayed(*self._app_root)
for an easy way to check that the app is succesfully launched (when the root element of the app is displayed)

I guess it could be done, like it is done here:
http://mxr.mozilla.org/gaia/source/tests/python/gaia-ui-tests/gaiatest/tests/functional/homescreen/test_homescreen_launch_app.py#60
60         Wait(self.marionette).until(
61             lambda m: m.title == self.test_data['title'])

This waits until the active app title is correct. Although, I'm not sure about that, because I don't really know what m.title is in this context.