Allow shorthand style for the repository field

If "user/reponame" is given as url in the repository field, create a
github-git-url from it.

Example:
user/reponame gets to: git://github.com/user/reponame

This fixes isaacs/npm#3783
Merge pull request #31 from robertkowalski/implement/npm-3783-shorthand-repo

Allow shorthand style for the repository field.