You can download it here, using my new tool... Oh, wait. Seriously though, go there, download the three files, and you're good to go.
Instructions are at the link above, but I'll duplicate the usage section here:
> github clone eric-wieser/computercraft-github ccgit Discovering files... Downloading: readme.md apis/dkjson apis/github programs/github [============================================] 100% > ls ccgit apis programs readme.md
Word of warning - github puts a limit on API requests (60 per hour, I think) - you may have problems cloning repositories with a large number of files and folders, particularly if github treats all users on a server as the same client.
This program also comes with an (skeleton) API to expose github information to other programs.