Update .starred? to take GitHub repository format

Update .starred? to take GitHub repository format (eg. 'rafalchmiel/octokit') while keep support for username and repository format (eg. 'rafalchmiel', 'octokit'). Warn the user that the latter is deprecated.
Refactoring by @pengwynn
Merge pull request #304 from rafalchmiel/deprecate-user-repo-format-of-starred

Update .starred? to take GitHub repository format