add test cases for bug 948826. 

create a privileged app for testing purpose

I cloned uitest and make it a privileged app.
All the changes made for test_apps/ is the manifest.webapp

After that, I created three test cases for audio_capture, geolocation and contacts.
Also, I added test scripts in tests/python/gaia-ui-tests/gaiatest/tests/functional/system/
 + test_privileged_app_audio_capture_prompt.py
 + test_privileged_app_contacts_prompt.py
 + test_privileged_app_geolocation_prompt.py

The test cases will make sure there's a prompt before the privileged app accesses the function.