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