Issue title: DiffURL & PatchURL is not available in PullRequest struct
 Issue body: I need to access the diff and patch urls that is being returned in the json structure but these attributes are currently not available in the PullRequest struct.

I will submit a pull request shortly that solves this issue.
 Comments: 
 Comment 0: fixed in #175
{'source': 'GithubIssues', 'repo_name_with_owner': 'google/go-github', 'issue_id': 65074280, 'repo_stars': 9509.0}