Stub Issue#closed? and Issue#commenting_users

Stub there methods in test to avoid getting error:

`GitHubBub::RequestError: message: 'API rate limit exceeded for my.ip.add.res.'`
Merge pull request #181 from mrgilman/update_schema

Update schema
Merge pull request #177 from ck3g/fix_issue_valid_for_user_test

Stub Issue#closed? and Issue#commenting_users
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Well, two actually. Re: Issue #135
Merge pull request #179 from philtr/one-click-unsubscribe

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add a model and migration and tweak some relationships