Add everything that’s required to run tests with a headless browser

@fesplugas what do you think? Let me know if you would like any changes.

- add gems capybara with poltergeist driver, as well as a gem that 
  integrates all (minitest-rails-capybara)
- new rake tasks „test:features“ and  „test:without_features“.
- the default rake task first runs test:without_features then features
- add one simple feature test for successful login (https://github.com/typus/typus/blob/7b1297cf36bb386402ad7f91c1e1ee8dfafeba17/test/features/login_test.rb)