Make poltergeist able to show debug web inspector

With the poltergeist_debug driver, you can now put 'page.driver.debug'
in the cucumber step definitions and capybara will pause when it gets
to the line and open a web inspector window of the tests in which you
can see the errors.

features/support/poltergeist.rb

@@ -1,2 +1,7 @@
 require 'capybara/poltergeist'
-Capybara.javascript_driver = :poltergeist
+
+Capybara.register_driver :poltergeist_debug do |app|
+  Capybara::Poltergeist::Driver.new(app, :inspector => true)
+end
+
+Capybara.javascript_driver = :poltergeist_debug