Add gist-embed 1.3

This library comes from https://github.com/blairvanderhoof/gist-embed and its source identifies it as being version 1.3.

It allows people on hosted blogging platforms like Wordpress or Blogger to easily embed gists. Hence putting it on a CDN will make it that much easier for bloggers.

:white_check_mark: 1 commit
:white_check_mark: 1 library
:white_check_mark: includes package.json
:white_check_mark: passes `npm test`
:white_check_mark: includes original and Uglified version
:hourglass: requested original author tag the release