services: GitHub: pull_requests API support (GET methods)
github: fix issue comment methods

Turns out that to get a single comment of an issue, you should not
specify the issue, but rather ask the generic issues collections for a
repo.
Merge pull request #36 from DmitrySandalov/github_pulls

github: pull requests GET API support
github: code review for pull requests API

Use different classes for a collection of pull requests and a single
pull request. Rename the `get_commits` method to just `commits` to be
more consistent with the `files` method.