Fixed og:type for profile and homepage.
Upped og:image dimensions to 200x200.

As per http://stackoverflow.com/a/15560644/504018
Added og:image logo option to profiles and homepage.
Added more metadata for open graph and html validation.

See https://developers.facebook.com/docs/reference/opengraph/object-type/website
See http://stackoverflow.com/a/7623986/504018
Merge pull request #2045 from patcon/2045-facebook-share-img

Appropriate images when sharing link to Gittip homepage/profile