Fix images in getting started guide.

We changed the error style, so they're all wrong. Now they're right.
:smile:
Getting Started Guide: strong_parameters

Stuff didn't work because of strong_parameters. Now the right calls are
in place.
fixing link to validations guide in getting started guide
Re-do Getting Started application with Rails 4.

The sample application with the Getting Started Guide was very out of
date. I've re-done it on edge (as of 51b9def5bf108fb566e) so it should
be good to go with Rails 4. It's also in synch with what the guide
actually says.
Remove unnecessary require

`require 'rubygems'` is already required in Ruby 1.9 or later.
Merge pull request #9026 from tricknotes/remove-unnecessary-require

Remove unnecessary require
avoid creating an object in every call

This was a suggestion of @carlosantoniodasilva, thanks!