tidy up features support files and install gems to support this

part of pivotal tracker story https://www.pivotaltracker.com/story/show/76232594
1. Addition of headless gem so capybara-webkit can be used without
   having to call xvfb-run as part of the cucumber command
2. create an env_local.rb with our configuration and move out of the stock
   env.rb file which can get modified or overwritten if cucumber is upgraded.
3. Get delete spec for admin/admin_user.feature to work regardless of
   javascript driver being used
refactored step_definitions to mirror app directory structure

part of pivotal tracker story https://www.pivotaltracker.com/story/show/76232594
implement README.md giving more information on how we use cucumber

part of pivotal tracket story https://www.pivotaltracker.com/story/show/76232594
refactored wording of features to improve clarity

part of pivotal tracker story https://www.pivotaltracker.com/story/show/76232594
reworded from format
Then I should be on the "Admin Admin User New" page
to format
Then I should be on the "New Admin User" page for the "Admin" role