AMP Library upgrade

[marcelovani]

This PR makes the amp library up to date with the latest ampproject/amphtml code

The validator-generated.php file was generated using this fork https://github.com/marcelovani/amphtml/tree/marc-generator

The fork is up to date with ampproject/amphtml

We are waiting for https://github.com/Lullabot/amphtml/pull/7 to be merged first to then issue a pr for the php-validator-generated branch

[karens]

@marcelovani I see you have been working on ways to upgrade this library or create a new one. I'm glad to see you doing this. I was wondering myself about whether the Wordpress plugin could be an opportunity to do this better. I'm curious which path you think has the most promise, trying to resurrect this library or starting over? I've been too busy to help much but am supportive of what you're doing.