[RFC] Fixes #19: Update title and meta tag text. Add Twitter Cards and Open Graph metadata

Requesting peer review since this metadata is hard to change after landing (both Facebook and Twitter cache this).

Changes made:
- Update existing meta tags to reflect the usage of this site.
- Add Twitter Card with big image.
- Add Open Graph metadata with big image.
- Add .DS_Store to .gitignore since I'm tripping over that.