Deduce a web page url from a github repo url

If `homepage` field does not exist, but `repository` field points
to a repository hosted on GitHub, the value of the `homepage` field
gets set to an url in the form of
https://github.com/[owner-name]/[repo-name]/ . If the repository
field points to a GitHub Gist repo url, the associated http url
is chosen.

Fixes #15