Removed capybara-webkit from the dependencies.

The Capybara gem includes Selenium, which should be enough to run acceptance tests in Travis-CI.

This should speed up our Travis builds, because the installation of the capybara-webkit gem took a long time on each build.
Added skeleton acceptance test for the authentication functionality.

It still needs some work before it passes.
Implemented very basic main page. Implemented some tests to verify authentication.

Some more tests are needed to test authentication functionality fully.
Added acceptance tests for the login process.
Implemented more tests dealing with authentication.
Added more acceptance tests for authentication. Cleaned up a bit Devise views.
Use helpers instead of hardcoded paths in acceptance tests.
Added simple_form gem with support for Bootstrap.

We will migrate all our forms to simple_form. This is intended to fix the breakage that currently happens when a form is submitted with validation errors (Rails form generator and Bootstrap are not fully compatible, causing an ugly form to be displayed in this case).
Give a little more space above notice and error messages.
Migrated the sign up form to simple_form.

It doesn't work as cleanly as I would like but it integrates well with Bootstrap and enables Bootstrap validation styles without breaking.
Added icon to the sign up form submit button.
Converted Devise new confirmation view to simple_form.
Migrated the Devise new password view to simple_form.
Migrated Devise edit password view to simple_form.
Migrated several more Devise views to simple_form.
Migrated Devise unlock view to simple_form.

This means all forms in Devise views use simple_form.
Added placeholders to form inputs. Added rdoc comments to custom inputs.
Changed label and placeholder in some password fields, specifically in those Devise forms that can be used to change a user password.
Added autocomplete=off to Devise views that allow editing profile data (registration edit and password edit).

Firefox still seems to autocomplete passwords in these fields in some instances, but I think it's Firefox being opinionated, not much I can do about it.
Added Nokogiri gem to test group.

We will use it to parse html content in emails sent by Devise during testing. This way we can test that e.g. email confirmation emails contain the correct confirmation link.