Remove title attributes

This pull request removes title attributes from `_s`

While title attributes are grounded in good meaning, they are not very useful. They're [used in many ways, but only helpful in a few situations](http://blog.paciellogroup.com/2012/01/html5-accessibility-chops-title-attribute-use-and-abuse/).

In the case of `_s`, they're used to provide additional information on links. However, this information can only be accessed via tooltip by mouse users. Screen readers by default do not enunciate the title attribute, so the title attribute isn't useful in `_s`.

The [WordPress Accessibility Team](http://make.wordpress.org/accessibility/) is working to [remove title attributes from core](http://make.wordpress.org/accessibility/2013/08/09/title-attributes-galore/) – [slated for 3.7](http://core.trac.wordpress.org/ticket/24766) right now after some [good discussion on it](https://irclogs.wordpress.org/chanlog.php?channel=wordpress-dev&day=2013-08-07&sort=asc#m658830).

Let me know if there are any questions!