Admin user feature

finished admin admin_user features including testing delete
    
    part of pivotal tracker story https://www.pivotaltracker.com/story/show/76232594
    
    CHANGES
    1. addition of gems for headles javascript testing (currently delete popups)
      - capybara-webkit
      - poltergeist
    2. Implementation of admin admin_user feature
    3. modify cucumber enviroment to use poltergeist (phantomjs) by default

Can move `Given I am a new, authenticated user` to Background