A plugin for sending 404 headers and an 'onPhantomPageCreate' hook for plugins to access the page object

I'm not sure whether this is something worth including in the main repository, but I wanted to be able to send 404 headers for non-existent routes in my Ember.js application.

I can keep my plugin in a separate repo but it would be handy to have a plugin callback for accessing Phantom's page object so that I can register an onConsoleMessage event listener.