GitHub: Add support for pull request Reviews

When doing code reviews for pull requests in GitHub, it is sometimes useful to keep comments in pending until finished reviewing (to avoid email spam). It is also sometimes required for a pull request to be approved by reviewers before merging. Currently SmartGit only supports posting single comments. Without any integration with Reviews (see docs: https://developer.github.com/v3/pulls/reviews/), it's a bit of a pain to manage pull requests when switching between SmartGit and GitHub (and I much prefer SmartGit for pull requests).