Modal delete confirmation dialog with boostrap, with Capybara JavaScript testing

 - a Bootstrap modal as a new partial _delete_confirm.html.erb, rendered
   in the Page#show view

 - *not* using the typical JavaScript to override $.rails.allowAction,
   $.rails.showConfirmDialog & $.rails.confirmed  to replace the standard
   confirmation dialog; instead, opted to make the visible delete button just a
   trigger for the modal, and the modal's "Yes" button is what is mapped the
   actual Rails delete method (with no extra confirmation)
   I feel it's somewhat simpler in this case when there's only the current item
   being deleted - there's no extra JavaScript needed at all, apart from Bootstrap's own.
   The downside is that you can't use this as easily for delete links e.g.
   on an index page, and also the actual delete method has to be triggered via the
   modal's button, which is maybe a bit ugly.

 - add poltergeist for Capybara JavaScript testing

 - add database_cleaner to handle test database cleaning with JavaScript testing;
   the required configuration is based on:

 - new config in spec/spec_helper.rb for both poltergeist & database_cleaner