Bump version to 6.1.3
Bump version to 6.1.3-dev
Add tags for opengraph/twitter cards; #510, #527

Facebook OpenGraph and now Twitter Cards are ways to share content
online and we get some degree of control over how content from
Gittip.com is presented in those contexts. This is a first attempt to
influence that. I'm going to deploy this, see how it looks, and iterate
from there.