Merge pull request #91 from sbesson/flake

Run scc and tests through flake8
Make UnrebasedPRs command inherit from GitRepoCommand

- Reuse GitRepoCommand initialization and main repository initialization
- Only propagate arguments of interests to the subfunctions
- Move merge_base() as a low-level Git function of GitRepository
Add logic to parse comments from PRs with no notes
merge master -D org -I include -E exclude: PR 92 (Unrebased PRs improvement)