Integration test helper

This is continuation of work by @srowley in this PR: https://github.com/NoamB/sorcery/pull/495

What I did was just adding HTTP verb option for login_user and logout_user (don't know if the one for login_user was needed, I've added it for consistency).

There was one more problem I've noticed: the param sent for user login was always `username` while I used `email` in my application. I made it a bit more flexible, but I'm still concerned it's not the best solution.