Add PullRequest.ismerged() method with retry decorator
Add method to check whether PR base repo has issues enabled
Return empty list in get_labels()/get_comments() if issues are disabled
Fix test_pull_request unit tests
Add unit tests for get_labels/get_comments with issues disabled
Fix recusrion in is_merged() method
Merge pull request #176 from sbesson/131_repo_no_issue

Teach scc merge commands to work with repos without issues