Fix scc to work with repos where issues are disabled

[sbesson]

The current implementation of scc, especially the PullRequest object assumes issues are activated by a repository and that each PR has a corresponding issue. By default, issues are disabled for e.g. fork repositories.
The code should be modified to allow commands to work with repos without issues, for instance we could catch the following exception:

GithubException: 410 {u'documentation_url': u'http://developer.github.com/v3/issues/', u'message': u'Issues are disabled for this repo'}