docs(index): check for $pristine in 'wire up a backend' example

Reference https://github.com/angular/angular.js/issues/1746

The docs on the homepage for the 'wire up a backend' example need to check the $pristine value of the form element before setting the error class or showing the 'required' help text.

**Before**:
![angularjs superheroic javascript mvw framework 2014-01-21 20-41-25](https://f.cloud.github.com/assets/28543/1970864/0fbffd9a-8317-11e3-8be3-1b26f287b579.png)

**After**:
![angularjs superheroic javascript mvw framework 2014-01-21 20-41-44](https://f.cloud.github.com/assets/28543/1970865/1afeab34-8317-11e3-8d3a-08f58af1c952.png)