Added Pinterest site verification file

• Added `pinterest-051f6.html` for Pinterest (business?) site verification.
	» More info: http://business.pinterest.com/verify/
Additional meta tags for Twitter cards

We'll eventually want this linked to an app card once we're live on the App Store.  Until then, this should give us the site verification we need.

• 3 new meta tags: `twitter:card`, `twitter:title`, and `twitter:description`.
• Existing `twitter:site` tag fixed to use normal HTML quotes, not curled quotes. Whoops.