Raise an error for invalid SE OpenID logins (includes live tests)

This simply checks for the presence of the `usr` cookie after attempting to login to Stack Exchange OpenID. If the cookie is missing, it raises a `LoginError`.

I added live tests to confirm that this produces the expected behaviour.

I added a live_testing module that checks for the presence of the `ChatExchangeU` and `ChatExchangeP` environment variables, instead of Travis-specific ones, so that we can also run these tests locally.