Added open_issues_count to REPO resource to match API output.

The REPO resource already contained `open_issues`, but didn't contain `open_issues_count`, which is also contained in the API output.
`forks`, `watchers`, and `open_issues` are respectively equivalent to `forks_count`, `watchers_count`, and `open_issues_count`. The non `_count` versions of these fields only exist for backwards compatibility.