Add page_title helper

Hi Winston, I would like to have `#page_title` in the gem. It would be helpful for lazy people :coffee: 
I also updated the README.markdown file. Sorry if I shouldn't update it :(
For the tests I intended to stub the `I18n.t` but it looks a bit weird. Could you advise a better idea to test it? Thanks.
You can test it like this:

````
it "generates translations" do
    I18n.backend.store_translations("en", {controller_name: {title: {action_name: "Hello"}}})
    view.page_title.must_equal "Hello"
````

This would make sure that it actually looks up the translation.