added scrolling animation

I made it so that when you click next the page will scroll to 300
pixels above the target element. I didn't add the ability to have
settings, etc because I didn't need them, but I thought I would give
this extension back to the open source world anw.
fixed error where last step would show up

The error was happening because I was trying to set next_tip_id where
there was no next_tip so .attr('id') was returning an error and the
cookie was not being updated to say that the presentation was finished
Update README.md

updated Readme
Merge pull request #3 from davidbyrd11/master

added scrollTo target functionality