The non-gender-specific singular use of they has been around for five centuries. It was good enough for Chaucer, Shakespeare, and Austen, and so it's good enough for me. Not only is you plural, but it's the oblique form of ye. One might as well condone saying me read book. Also, they (as well as she) is a borrowing from Norse. The original third person plural pronoun in Old English was hie which was him in the accusative. The pronoun stuck around in colloquial speech, though. When people say things like that'll teach 'em. The apostrophe is probably also a mistake. Many moons ago, I wrote this blog entry on it.

Language does not change because of laziness, foreignness, or youth. It changes because the people speaking and writing it use it in different ways so as to facilitate better communication.

Ceci n'est pas un seing.