Add custom service for Gists, use GitHub API and set additional metainfo.

It populates Gist-specific metainfo like ProjectURL, BrowseURL for
files and root folder, and LineFmt.
It uses GitHub API rather than git commands (thus it'll work in Google
App Engine).
Use the file contents provided in first GH API call.

There is no need to fetch the files individually since they're already
provided.
Use gitHubCred for GitHub API call; fix LineFmt.
Use expand to contruct complete gist URL.