New provider support: Latch

Hello. I've added a new provider, Latch, to the additional-providers module. This is just a single file, and it follows the Stackoverflow/Yahoo OpenID simple form. See latch-app.com for an overview of this new service if you're curious.

Not sure how to add a new provider info to the userguide on sourceforge. I posted a question to the forum on that. If you have a procedure on that, please do let me know.

Thanks!