Auth via user password

* Deprecate selenium in favor of poltergeist. It's faster, consumes less memory and, most importantly, does not show lots of useless windows; I had to make this change because HTTP authentication does not work very well with Selenium;
* Use `User#password` to authenticate users
* Deprecate `early_adopter_name` in favor of `User#first_name`
* PreviewsTest was causing some issues with poltergeist and I had to change the test slightly;
* Add `GET /logout`;

/cc: @exalted