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.
Fix inconsistencies in terms of new lines, et al
Merge pull request #305 from rafalchmiel/fix-inconsistencies

Fix inconsistencies in terms of new lines, et al
Refactoring by @pengwynn
Merge pull request #304 from rafalchmiel/deprecate-user-repo-format-of-starred

Update .starred? to take GitHub repository format