Fetch all pages of github comments

Comments on github issues, which covers pull requests, are paginated
in the github api. Update the comment fetching to get all pages by
navigating the link headers.
Merge pull request #18 from bz2/github_multiple_comment_pages

Fetch all pages of github comments

Comments on github issues, which covers pull requests, are paginated
in the github api. Update the comment fetching to get all pages by
navigating the link headers.