Add instructions for installing PhantomJS on Ubuntu via npm
Configure Poltergeist for driving JavaScript tests (uses PhantomJS headless browser)
JS tests happen in a separate thread that doesn't share the db transactions usually used in tests, so dont use transactions for JS tests, use truncation db clean strategy.
Added poltergeist to Gemfile test group and cleaned up Gemfile a little
AJAX test now passing by flagging it as a JS-dependent test. Previously it failed as it was using Capybara+RackTest that isn't a JS-enabled testing environment. Now runs using Poltergeist+PhantomJS thanks to :js => true option.