Persona sample tests and bugfix

Good day.

I have slightly modified the Persona sample. So, now it:
1. gets ``audience`` parameter from config file rather than using ``const``
2. uses ``localhost`` as a default ``audience`` parameter so the app works by default now
3. checks the response from **persona verifier** rather than just parsing it (which would lead to a possibility of using assertions having being issued for other sites)

Moreover, I have added a demo of how the app which uses Persona can be covered by tests.