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.
Thanks! I'm updating the test.